IP Ban Possible?

[NeroObsedian]

I have roughly 15 accounts created from the same IP and an almost the equivalent list of IPs that have been hitting my site with bots. All of them resolve to the same ISP.

Problem I'm having with this is that the bots that are making it through, are posting adult advertisements on my boards. I recently updated my CAPTCHA and it seems to be holding up, one may make it through here and there, but the ones that are doing it all resolve to one ISP, several of which to single IPs.

I looked through some options, the forums here are getting upset because minimum search strings so I have to ask, is there a way to block and IP or IP range built into vbb or is there a hack out there which will deny them?