internet filter

I have a client that has two mac stations for internet access in an upscale wine and spirit store. He has his home page set to a site to compare wines. The problem is that he has had a couple of idiots that want to check their email or visit porn sites. Any recommendations as to what is the best way to curb this???? I thought an internet filter would be a good idea, but any suggestions would be helpful

What browser are you using? Internet Explorer has a Content Advisor you can setup and needs a password to bypass.