text limit

Hi all, I'm wondering if it is possible to set a limit on the number of words it takes to make a valid post.
Say 5 or 10 words, so that people cant type something stupid like "askldfh;skljdhf" and submit it as a post, or just put a href post up as an ad.   I realise it wouldn't completely stop the problem, but it would help.   Does anybody know if this feature is do-able, or have a code for such a feature?

Cheers