I havenever found filtering software which you could not get arround, over or through. I think the best advice given here is talk.
Any 14 year old who is told outright that "I can run a report and see every web site you visit just like that!" will self regulate his internet access.
However, if you do go down the filtering rout, then I would also look at blocking all anonymous proxies (If not already in the software) that you can find, allong with things like Expat Sheild (Which I use to watch BBC iPlayer from Australia) becasue that uses VPN technolgy to create an anonymous connectionto a UK server. In fact, you could look at disabling the built in VPN software on the computer, that would block many of the get arrounds.

