There are some low-hanging fruit that the system is already looking for, like duplicate posts and posting too quickly and short comments.
A more creative example is 4chan’s robot9001 board (nsfw) which just blanketly prevent users from posting ANYTHING that has been posted in the past. - I don’t think we need to go to such an extreme with discourse, but the server can do a lot of things to prevent crappy content.
What I am saying is that it is not enough to simply dis-allow it, but instead to encourage people to post better in general so they aren’t compelled to get around the limitations. Its a peopleware problem.
@sam was already talking about Bayesian Analysis as a extension. This is what they do to filter out email.