[CentOS] Define HTTP proxy system-wide (Squid and Squidguard) ?

Hi,

I'm currently configuring Squid and Squidguard for filtering web access 
in a public computer room. So far, everything works fine.

I know how to configure Firefox to use a proxy, but there's no way to 
protect that configuration, e. g. a more knowledgeable user might 
circumvent this configuration. Is there some way to configure an HTTP 
proxy system-wide?

Cheers from the sunny South of France,

Niki