Hey, so I’ve been looking at this problem myself in the past couple days.
I’m finding that WP blog comments that are held for moderator approval are not getting inserted into the activity stream, but blog comments that are auto-spammed (from the comment-blacklist), are showing up in the activity stream.
First caveat: there may be a simpler way to do this, such as disabling user blogs in the wp-admin backend or something (I don’t know if that would actually remove ‘blogs’ from the listings or not), but it sounds like that might not serve your purposes entirely anyway.
That said, to modify what appears in the admin bar you should take a look at this how-to: