How to Block Ubuntu Changing Proxy Setting

[Avin]

Hi All

I have  two problem plz try to help me out:

      1)  How can I block the Ubuntu user to change Network setting (Proxy setting). They able to change the browser proxy setting and bypass from squid filtering. how can I restrict user to changing proxy setting.
       2) How to block All messenger (gtalk, skype and yahoo messenger) through Squid ACL.

Thanks in Advanced
Avin
         

[Ricky]

Solution is to use Transparent proxy and then block all other ports apart of mail and HTTP.
After that block servers which are used by above services.

You can find solution in this forum given already.