How to force network operator selection once and for all?
APN Lock may work.  It appears to give you the ability to block certain networks APN's which won't affect your ability to use Voice or SMS but won't allow your phone to make a data connection with the networks you block (by changing the APNs so you phone fails to connect).  I'd be careful though, messing with the APNs can cause problems this app doesn't have any comments yet.