Stopping spam bots account requests Honeypot
I had a problem with stopping spam bot account requests, hundreds weekly - I had removed comment or account requests from the sites but still got loads of spam. I had installed CAPTCHA which reduced this a bit but I was still getting lots of requests so I installed Honeypot and it did the job.
Honeypot uses both the honeypot and timestamp methods of deterring spam bots from completing forms on your Drupal site (read more here). These methods are effective against many spam bots, and are not as intrusive as CAPTCHAs or other methods which punish the user [YouTube].
Video - From the book "Anything You Want" by Derek Sivers - See http://sivers.org/a for more info