stop downloading files for microsoft isa server 2000 clients

I have network of 250 users, and I have 256k link for internet browsing. I have ISA server 2000 for internet sharing as an integrated mode. I want that my client could not download any file, but I want them only to surf/browse internet. I even disable FTP protocol from protocol rules also I disable zip, rar, exe etc extensions from content group portion. But my clients still downloading files. I tried my best to stop them from downloading but all in vain. Please help me in that my clients could not download files. I will be very thankful to that person how will help me in that.