Forgot to mention, I don't think its actually true bots, but a person registering an account (validly, with user interaction), and then turning it over to a script to post a few times (or just copy/pasting a few posts in)
Probably the only way you can slow them down is to set account activation to admin.
This means more job to do for you and if they arent using any obvious "spammer" nicknames, then you cant figure whos going to send spam and whos a good person.
Maybe you could find out whos their ISP and send some email to abuse@their_ISP.com (or whatever their ISP's contact is...) and ask them to do something.
It's a long shot, but worth to try.
You cant keep banning IP's forever as this might afect your other registrants aswell.