how to force the use of a web proxy?

[Kevin Kempter]

Hi List I have FC6, FC5, CentOS and Mepis boxes in my network. I have a proxy 
which filters web access. I want to be able to force the entire network to 
use the proxy. Currently I've simply gone to firefox and added the proxy to 
the connection settings - however any user could change this back to direct 
connection if they wanted to...

How can I force the network for each box to use the proxy and basically not 
work at all if the user tries to use a direct connection to the internet?


Thanks in advance