How to prevent illegal activity on squid?

[Al - Image Hosting Services <al@xxxxxxxxxxxxxxxxxxxxxx>]

Hi,

I have a squid server setup to block objectionable content from the  
web. It is publicly available, and free. All anyone needs to do to  
use it is signup for a free account. The problem is that so far  
everyone who has signed up either wants to send spam through the  
squid proxy, go to sights that are banned in the country of their  
origin, or do something else illegal. What are all the different  
things that squid can be used for that are illegal and how do I  
prevent people from using it for that?

Sincerely,
Al