I get a lot of spam bots visiting my site and trying to post comments.
akismet stops them being in my approve queue (i have to approve all of the other comments)

but I wanted a way to stop the bots being able to register.
I also wanted a way to remove all of the old [inactive spambot] accounts.

so here are two wordpress recommendations

wordpress user spam remover
– automatically deletes inactive [never used] accounts older than the specified number of days
it removed 14000 accounts from my site :|
and

wordpress reCaptcha
it adds a recaptcha to the signup, comment and login boxes.
it might make real people less willing to comment, but it also makes spambots bork.