If a hero is broken out of mind control, they cannot be controlled again. Heroes that are controlled do not attack other heroes, only board the spaceship. Does this mean that if a single hero breaks mind control, the heroes automatically win, with the freed hero just bashing the spaceship?
No, a hero breaking free of mind control does not itself make the game unwinnable for the traitor.
The Traitor's win condition for this haunt is exactly as follows:
You Win When ...
... all of the heroes are either dead or on the spaceship.
Note that if either of these happens to a hero, they are out of the game. Effectively, this means that the traitor wins when all heroes are unable to act. The "can only mind control a hero once" limitation simply means that any hero that breaks free of mind control must be killed for the traitor to win.