[sv1cec]

You could add a little piece of code, to add the bot's IP address to your banned IP addresses list, but the question is why?

The bots will not come back as humans, to register in your site. Their ... business plan is to attack as many sites as possible in the shortest possible time. That's the same reason, why they do not make their programs wait for 60 seconds, before hitting the "register" key.

I had such a routine in my antispam filter, in vB 3.0.xx. More than 60,000 IP addresses attempted to register (yeah, I was saving them in a table), in two and a half years (March 2nd, 2011 to November 23, 2013). Several of them came back more than one times. Always they were stopped by the "too quick registration" filter.

Unless you are prepared to run a special consolidation program, to identify similar IPs, it's not worth it entering the bots IPs in the banned IP addresses list.

What I would suggest, is that you use more than one traps for the bots. For example, this one, the one that checks the IP/email of the registering person against known spammers databases, as well as other traps. For example, use a special field in the registration form (for example, the registering person's country). Most bots do not bother entering that correctly. If you compare that field, with the location of the IP address, you can trap these bots.

So far, in my updated (vBulletin 4.2.2) site, about 98% of stopped bots are trapped by this mod, another 1.8% is stopped by comparing their IP/email to StopForumSpam database (there is a hack for that) and the rest .2% is trapped by my other filters.