[In Omnibus]

Admittedly, I got off on a tangent there.

OK:

1) Verify e-mail addresses. Do not allow duplicate e-mail or IP registrations.
2) Require PCRE regular expression usernames.
3) Utilize the Question And Answer registration feature. Use questions and answers only someone with some level of knowledge about your forum specifically would know without having to look it up. Like who or what or how many of something or what colors appear in your site banner. Bots can't answer that.
4) For new users, make any post with more than 0 URL links go into moderation. No user should be linking to anything in their first post. You can also create another usergroup and promote new users to that group after x number of posts if you want to give users the ability to post links after enough posts to know they are not a spammer.

All of the above can be done without installing a single modification.