How do I prevent a second player from entering an area occupied by another player?

I am building a gambling machine that uses command blocks to take money from, and give money to a player. It also sends game messages to the player via the command blocks. Therefore, I want only one player in the gambling machine area at a time.

Is there some way to read a flag at a distant location that might indicate a person is there (because they are standing on a pressure plate)? I tried using pressure plates the player stands on, but when the player leaves, the redstone circuit "freezes" so it thinks a player is still gambling. Is there a way I could communicate the information wirelessly?