Each of our employee's reads and signs an internet usage policy before we even give them internet access. This basically says that they won't try to download anything stupid, they won't use the net to try to look at porn, drugs and such like and they will only use it for work related purposes.
We have had two people who have tried to look at porn and therefore sacked as they have breached the policy that they signed. We have also had to warn several other people who have tried to bypass our internet blocks.
At the end of the day, we run a business not an internet cafe.