Blocking Proxy Softwares from accessing Internet by using ISA 2006

Hi;
How can we prevent Proxy Softwares like Hotspot for example from running and accessing Internet. We're using ISA Server 2006 and I discovered that one of our employees accessing some websites those are blocked in ISA by using a software I don't know its name. the user is a limited user and he can't install any software but i think this software is running by another way.
any help please.

Thanks