I noticed that the bots (that came to my site) always started a new thread. So I setup the Registered group to Not allow the new user to make a new Thread, only reply and I prevent them from posting links (vb.org). Then I setup another group of regular members with full permissions. Then I setup a promotion from the Registered to Regulars. The promotion said they needed 5 post and 5 days but you can pick anything. I then setup an automatic welcome PM to inform legit users of the restriction. This has been setup on my site for several months now and I haven't had a single bot make a post. Plenty register (and from email domains I've never heard of), but they don't post. Every so often I go through and prune off users that never made it from Registered to Regulars.
