I have made a small / simple / nasty but effective, php script that allows easy ´weighing' of a comment.
The idea is to handle spam. most spam in fora is rather easy to recognize as most spammers use the real words to allow for search engine hits. the code is here http://www.pmwiki.org/pmwiki/uploads/Cookbook/commentdb_1_1b.php it is part of the comment DB add in for PM wiki.

What whould be the best way of integrating this in YaBB, It should somehow link into the 'post' code.

The routine works by scoring words based on a static weighted list, if the score is to high you eighter put the post in 'detention' or produce an error.