Looking for a Spam preventing Captcha or mechanism for Buddypress plugin enabled wordpress theme

hi there, i am looking for a mechanism to ensure that bots / spammers cannot create user ids on my wordpress site enabled with buddypress. i tried to install splogs here from wpmu and it says i require a MULTISITE.

any suggestions or plugins i can use please to ensure that the sign up process is
1) double optin if possible
2) human readable captcha like fun captchas can be implemented.
it doesnt look like wpmu has that here on the list of plugins. unless i am missing something.

thanks
cynthia