Find some lua script that has a specified 'admin area', or forcefield. This lua script you find shouldn't be limited to just certain maps, though even those could help.
That would be a good start to look behind the scenes to detect how lua determines if a player is attempting to cross a border or 'box'.
Then you could review the gmod lua wiki to determine how to play sounds that only the player can hear, or a sound entity that can't be heard outside the border.
Just some ideas of where I'd start looking.