What will happen if civilian ATC suddenly sees unknown plane which request landing due to emergency. Can ATC just forbid such land? If yes, what they do? What if it's foreign military plane?
ATC can't "forbid" anything really, they certainly can't enforce it - there is physically no way to stop an aircraft landing. Notwithstanding hostile intentions, in any sort of emergency ATC, civilian or military, will do everything they can to aid the safe landing of an aircraft in distress.
Remember, ATC are not "in charge", they are there to facilitate the safe, orderly flow of aircraft within the rules set out.
As for what would happen once the aircraft is on the ground, that's a whole other matter not really to do with ATC at all.
There is the incident of Viktor Belenko who defected to the West and landed a MiG-25 at Hakodate, Japan. There is even a book ("MiG Pilot: The Final Escape of Lieutenant Belenko") about his life and defection. As I remember, his radio didn't handle western ATC frequencies, so he couldn't even declare an emergency. He had a close call with a airliner who was taking off, but was able to land safely.
Stopping it would require a surface-to-air missile or something roughly equivalent.
A scenario where a foreign military plane would enter the airspace of a sovereign state without its consent is a bit far fetched, although not entirely impossible.
Should the said plane declare an emergency, the ATC most likely would not, and more importantly could not prevent a landing. if the airforce and/or air defences of the state in question disagreed with the landing intentions, they could intervene and transform the landing into a crash.