Only connect to certain Cell Networks

Hi all,

I need a way to block my phone from connecting to the AT&T cell network. I live close to the Canada-US border and ever since Fido launched it's national roaming network (Fido-EXT), I've been connecting to AT&T also because I have to have the phone set to "Connect Automatically" so I can connect to the main Fido network and Fido-EXT. Any ideas?

Thanks!