We had a very shortlived hit on our site forum a while ago and a simple call to the host had them put a scrambled puzzle on the registration page and there ended the problem. NEVER saw it again.
We have rebuilt it on our main site and still no worries at all.
It is a very simple add in, just a couple of clicks and it is there. Brilliant!
Occasionally, some clever dick may push his own product into the posts but that we jump on when we offer to charge them for their advert. They just get the 30 seconds treatment…. we spray and they walk away! LOL!