If you are on BP 1.5 go to the page admin and change the URL for register to anything but register or signup. Example: get-your-account
Use a captcha plugin that requires you to answere a question correctly. The number letter ones can be hacked
Make sure the words WordPress or BuddyPress is not shown in the footer or header. View source of page and then find and eliminate those words. Bots search these and then find them. You may also want to change the default text in sidebar.php about creating an account.