limit hero for players

Hello, I'm making a multiplayer map where you can choose a hero (by hiring it), but the place where is choosed is reachable for the normal units, so instead of make it unreachable I wanna know I can make it impossible to hire another one just for that player. I tried adding techtree requiriments...

Hello, to be concrete, I am having the following problem. I have a own map where the number of players is 5 vs 5, where each player has a choice of three heroes each .. Now what I'd like to know In the event that the game starts being 5vs4 players lack one .. as I can make a detonator to alert...