You could try and throw and old computer running Smoothwall or IPCop in between the modem and the users, then throw on Dansguardian addon for true content filtering, not only filter websites but also file extensions. Works a treat and highly configurable. You don't need a computer science degree to set it up either.

On top of that, it logs all traffic, so if people have been a bit naughty and trying to look at things they shouldn't, - time, IP and url should be enough to show then the errors of their ways.

You can administer the thing remotely, what more does one need