Here is an approach to the problem! You already have the abilty to find out when someone has been using the internet for someting other than business related subjects right? Then start docking them for usage (1 hour each time). Note: Before you can deduct pay, you have to inform them in writing that you are going to do so and get them to sign it. Keep this in their personell files. If you are lucky they will get the hint. A blocker program can be worked around. But if they could work around the blocker, they would be able to cover their tracks!
Just a thought. Tigerflight
Everyone has their two cents worth. Lets all try to donate our two cents and maybe we all can come up with a Dollars worth of advice!!!!!