Comment First of all, it's a losing battle (Score 1) 454
I worked for a company that sold web filtering devices primarily to schools. The school admins spent more time hunting down proxy sites, web proxy sites, figuring out how to block kids running SSH tunnels off their home PCs and tunneling with putty on a USB stick. The web filter did awesome, until you got one smart alec in the mix and taught everyone else how to bypass it. THEN you start in on locking down the PCs with GPOs, adding layer 3 filtering for external proxy sites, prohibiting any unknown executables from ever running (yeah, makes those self-extracting printer drivers fun).
Glad I'm out of that business.