Country change time in the Early, Mid, and Late War arenas is 1 hour.
If someone wants to call out a mission to the other country, and they really are hard core about it, there is no way to stop them.  Cell phone, text messaging, Ventrilo, and so on are just a handful of ways to get the message to a buddy.
On the other hand, this issue is not as widespread as many players thinks it is.  There are a number of players who are incredibly paranoid about this.  i.e. "If I get shot down in a mission it has to be spies that caused it!!!"
I am not saying it does not happen.  It does, but it is doubtful anyone could devise a system that can prevent it.  Give it your best shot.
Currently, the best way to stop it, is to shoot down the other guy before he shoots you down.  Ta-da!