We already have an unofficial spam detection system.
There's a bot called SmokeDetector which is maintained by some users that checks every new post on all sites using a great load of filters and if it spots a spammy, offensive or gibberish post, it notifies the community through some chat rooms. Our users can then check that post and flag it as spam if necessary.
A specific number of spam flags (5 or 6 IIRC) then lead to automatic deletion of the post.