you get a lot of spam bots here? i never noticed, but i dont venture out of the eris forum much.
for spam prevention i highly recommend using using both recaptcha and human verification on registration. pretty sure vb 3.8 has this built in.
this is like a brick wall for spam bots.
i see you use human verification already, add recaptcha and that should help a lot.
limiting posting privledges will help no doubt but at that point it is kinda too late, the bot already registered and now has used up a username for no reason....it will try to post spam and might succeed, just the link will be dead, you will still need to clean up te posts.