I agree there.... one word posts are terrible...
As a really simple method of doing it... (they will still be able to edit it and make it short, but I did say simple!)
Post.php...
Search:
if (strlen($message) > $MaxMessLen)
fatal_error($txt[499]);
Replace:
if (strlen($message) > $MaxMessLen)
fatal_error($txt[499]);
if (strlen($message) < 15)
fatal_error('Your message is too short. Please make your posts meaningful, so as to contribute to the community.');
I didn't bother to test it... but if that doesn't work I'll eat my hat

. (wait... I'm not wearing a hat....)
[edit: Blah! 5 seconds... I had to read it over to make sure it would work.]
-[Unknown]