Protecting a small network

We have a network of four computers in one room at our mobile home park. Three of us have admin rights via a password, and everyone else logs on as guest. How do we prevent unknown individuals (underage, we believe) from d/l objectionable images? We just recently installed a keycard system which should stem the problem, but would like to know your answer anyway. Thanks esconsult1.

This is what you need to do, just put up a hidden video camera your sure to catch him/her then.

Thanks; we have discussed doing that as a last resort. Some people object that it's too much like Big Brother. We'll see what happens with the keycard system which logs everyone who enters the building, but still doesn't indicate which room they might be using.

Change everyone's password, make sure you change the router password (if it's wireless). Tell everyone that the computer room will be locked up unless the activity stops.
For kids under 18 you are big brother.