Also consider designing your mission so that if the players want to do their objectives, they then are practically forced to stay on the area you wish them to stay at, if they want to be able to complete their objectives. This doesn't necessarily need death zones or the like. This would most likely mean that you would have some time-limit based event in your mission which would mean that if the players have not done something before the event, the mission becomes extremely hard or even impossible to accomplish.
Example: destroy all enemy aircrafts at airbase X before a large number of enemy reinforcements arrive. Now, if the objective is not completed before the enemy reinforcements arrive, the mission would become very, very hard, near impossible to complete.
I would try to use such design tactic instead of using artificial constraints.
In Capture The Flag (CTF) or Conquer & Hold (CH) and similar missions it is a different situation and certain zones (bases of each side) must be protected with artificial constraints.