kaled - 4:55 pm on May 29, 2010 (gmt 0)
How would you populate a black list? Where would you even start?
When a post with a link is classed as spam (by manual review) the domain could be added to the black-list. Of course, someone might try to deliberately get another domain black-listed, but the site would not be harmed in anyway so it's not likely to be a frequent occurrence.
It's also worth noting that, in theory, Bayesian-type spam filtering (as used by email scanners) could be applied to forum spam. I wouldn't suggest writing this from scratch, but maybe there's some code out there that could be adapted. Then, if a post score's too highly, it could go into pre-moderation.