Blocking Proxy sites on ISA 2006

I'm one of the administrators of a pretty good size Windows network (100+ users), and we are having a issue with Proxy filters. We run Windows 2008 Servers (DC1, DC2(ISA), and Exchange), with both Windows 2008 and XP workstations. My issue is that users are using proxy filters to access sites like myspace,facebook. Is there an entry i can apply to our ISA server to block these proxy filters (ex. sureproxy.com are a few)? I've tried to just go in and block them one at a time, but there are millions of these sites that offer this service, and i just can't block them all.

Any assistance would really be appreciated