Quote: "You could stick a script into a trigger zone that moves the player back each time they try and enter it?"
I'm glad to see someone actually thought of that.
I've done it and it works very well for blocking the player. All you need to do is move the player in a certain direction (The opposite of the direction you DON'T want them to go) and make it move them about 60 units (which is the fastest the player can move while running.... at default speeds)
The way I did it, I made four scripts, one for each direction to block the player.
The one and only,