Does NetworkManager support blocking non-home networks (UMTS/3g)

[nodata <lsof nodata co uk>]

Hello,

I can't seem to find it in the GUI or in the documentation.

Can NetworkManager be configured to only connect to home networks? Say I 
am on Vodafone and get free data usage, but then move to an area with 
poor signal for Vodafone, can I prevent switching to T-Mobile which 
costs me money?

If yes, can this be determined automatically, or is this done with a 
string match?

Thanks.