Proxy - How enable user offline to remove proxy settings

Hi

I have tied down my users to use a proxy server on their laptops. The
problem is when they go home and use their broadband they cannot remove the
proxy settings.

I am ok for them to be able to bypass proxy while at home but as soon as
they reconnect to the network they need to have the proxy automatically set
back on.

I do not want to give them any local admin rights.

Any ideas?

Thanks
Ian