this is an easy hacking technique, i’ve done that 3 times yesterday when trying to create users/blogs…
you can delete these users by going in the _signups table… the problem is that WordPress is not taking into consideration the registrations that are not completed, they store them in the signups table and they can not be reached when you check for users… so when a user create an account with a blog, the whole process is created but not verified… you can then visit the site without being logged in and without a trace.
WP 3.0 is different in that technique… but i suppose we could find a tweak right now.
Sounds like someone exploited a WordPress vulnerability on your site.
Are you using the latest version of WPMU / BuddyPress?
If so, did you upgrade?
You might want to read these posts:
https://wordpress.org/development/2009/09/keep-wordpress-secure/
https://codex.wordpress.org/FAQ_My_site_was_hacked
stwc’s summary of methods does seem to stop a lot of spam, but I’ve still been having some. I tried SI Capthca (https://wordpress.org/extend/plugins/si-captcha-for-wordpress) but that seemed completely ineffective.
My latest weapon in the war has been to modify Invisible Defender (https://wordpress.org/extend/plugins/invisible-defender) firstly to make it work with the buddypress registration page and secondly obfuscate its hidden fields by giving them random names and values:
http://bcbc.co.uk/mu/blog/2009/12/11/wordpress-registration-spam/
I think I ended up commenting out the email notification before first activation on a live site. I then added it back. If I remember correctly you also have your activity stream somewhat spammed so that might be another thing to look at 
@Andy Peatling
Ah…sorry to spam you somewhat but I was also wondering if there were any plans to enable easy “quoting” within the forums with this theme/BP version (or a future theme/etc). We’ve finally convinced ourselves that flat forums are good but I think we’re definitely going to have to get some sort of quoting mechanism working.
edit:
I’ve just seen this post: http://testbp.org/groups/buddypress-testers-614548248//forum/topic/forum-or-wire-with-comments/
Hopefully I’ll be able to get that sorted as a plugin sometime soon.
I’ve determined that the warning is generated when /wp-signup.php is accessed (mostly by spam bots). Can’t find white spaces anywhere. Is /wp-signup.php supposed to redirect to /register or to a blank page?
My site and the other listed both display the Registration Form and the PHP Warning: Cannot Modify Headers:
http://ttacconnect.org/wp-signup.php
http://memomu.com/wp-signup.php
These sites result in an blank to semi-blank page:
http://startupweekend.org/wp-signup.php
http://nourishnetwork.com/wp-signup.php
http://morgansjourney.org/wp-signup.php
http://poetrypress.org/wp-signup.php
Should /wp-signup.php result in an blank page or the registration form? Will resulting blank page eliminate the ‘Cannot Modify Headers’ Warnings in error_log? What is the fix? Thanks!
Plan for the upgrade and pick a time when your site traffic is low.
Let your users know ahead of time that you’ll be down for maintenance. Even with just a few users, letting them know is a courtesy, because if they do visit while you’re upgrading and they don’t know what’s going on, they may eventually leave because they think things break all the time.
Yes, we backup ahead of time. Really. With large site where it would be impossible to ftp things without it taking all day, just backup files in a different location on the server. Optimize & clean up the db while you’re at it. Who wants to back up spam?
Some of us with large sites do not deactivate all plugins and then reactivate later. With hundreds or thousands of blogs, it’d be a nightmare.
At some point when your site is very large, you’re just going to have to get used to doing some things command line (ssh). In many ways, it’s easier.
The moving of the them only occurred during BP 1.1. Shouldn’t have to do it next time. But! Paying attention to core changes helps you anticipate these things in advance. You have to do your homework.
The maintenance mode plugin may not work properly in MU. My fave trick, if I*really* wanna keep people out, is to toss an index.html file in the root. I can still get in the backend, but many users just can’t figure out they can type in these things without a link there. 
Man, this means I have to write up a blog post about it now, doesn’t it?
I just tried that as I am out of alternatives…
Just noticed when testing the new signup slug that the user gets an email with the following text:
“You can log in to the administrator account with the following information:
Username: test
Password: bd36dc14
Login Here: http://test.biketravellers.com/wp-login.php”
? : Why does the user get a random password sent as he alreaady chose a non-random one? This random one does not work by the way.
Is this a result of the spam procedures or a regular bug?
Cheers, Bike
Glad to hear it, levin! Hopefully that’ll hold the floodwaters back until the next generation of bots finds a way around it.
Tried @stwc change register-slugs suggestion, zero spam registration in a week! thanks alot!
I have had total cessation for the last two weeks without using plugins, using the procedure I outlined here.
Would be good if you could post ideas and solutions here too: https://buddypress.org/groups/fighting-spam-splogs
There’s already a few plugins etc mentioned there that might help you out too.
Andy, the username/email gets randomly made with every signup, so that won’t work.
Maybe this’ll help too: http://perishablepress.com/press/2009/03/16/the-perishable-press-4g-blacklist/
Also – make sure you are marking the users as spam not deleting them. This will block the username/email from logging in and/or signing up again.
They should just use activation mail again, works the best.
There already exists a topic concerning these spam sign-ups. Haven’t had any problem with this yet, but that’s most likely because I have my wpmu/buddypress/bbpress in dutch..
@Andrea_r No joy. Tried the # BEGIN ANTISPAMBLOG… code and just got another bot registration “terrancecline1973” a moment ago.
Thanks Sven, I will try to install this and have a look……
I changed the mail address from noreplay to a real one.
That fixed the spam problem for me.
See this plugin:
https://wordpress.org/extend/plugins/mail-from/
Thank you !
But what is the name of the file at which I can change and customize the text of the “confirmation e-mail ?
Customise the email message with specific text for your site so it is less likely to be identified as spam?
Thanks so much.
I’m using the object-cache.php file from Donncha as the only caching that I’ve installed. This seems to be the much of the space – If I drop these, will performace be slower, or will data be lost? Would you suggest the wp-super-cache instead?
I don’t have many blogs – one, basically, (the main BP blog, but it has 10,000 posts, but they’re not really posts, long story). It’s a complicated integration where I’m using BP for the community side of a larger site with 123,000 members migrated into WPMU/BP.
There isn’t much spam do deal with, thankfully.
Thanks again for the info.
You shoudl back it all up, yes. But you don’t necessarily need to ftp it down to your computer. I normally make a copy of the folder right on the server.
If you are double-sure you have cache files, you can clean them out. What are you using the cache? Because wp-super-cache has a cleanout button back there on the admin screen.
It’s not unusually large, no, dpeending on how may users and/or visitors you have. Lots of users, lots of hits, means lots of cache files & loads of upload files if they like to post pics to their blogs like you’re Flickr. (Hi mom!)
Yes, you’ll want to backup the db as well. Optimize those tables, and if you’re really feeling frisky, clean out the spam.
Remember that with the BP upgrade, the theme changes location. The MU part should be smoother.
Pick a time when your site traffic is low, like the weekend.
@stwc
Thanks for your effort, i just put it into my site, hope it can stop the spam registration.
