So no, you can't block them from joining you. You are, however, more than welcome to meet the ruler of the other realm in the region and hand the region back to them.
Eh the problem with this is how it damages your stats across the realm when you do this. Maybe regions that have joined for less than a week shouldn't do that? I mean Tamilak switches hands three times in 3 turns. I am not sure you want regions switching realms that often.
