I have seen 2 different solutions to your problem, but guess that there are many more. The first is checking to see if it is a spam bot at registration. You can find that modification
here.
The other is to make sure they can't post any links until they have a certain number of posts. I have cleared up about 97% of the porn and viagra links coming into my site. You can find that by going
here.
I hope these help you.