Well... that sucks, finding mat hackers is really difficult and hard to prove.

Couldn't you just make a white list like this?

Code:
//To prevent mat hacks.

materials\...                     from_steam

//For custom skins and models

models\player\...                 allow_from_disk
models\weapons\...                allow_from_disk
materials\models\weapons\...      allow_from_disk
materials\models\player\...       allow_from_disk
It would make the server's first start up take a little while snice it has to generate the CRC's for the materials folder but It would be very much worth it.