@xeon yeh thats true, I hadnt thought of that,
disabling the controls may not be the best idea, nor turning off the monitor, I think the best way would be to diable the buttons, so you can move around the games list and watch video previews etc, then you have to insert a coin to continue.
the obvious choice here would be to use a pandoras box 4 which is meant for this kind of use, but if using the pi then the only other thing I can think of is to use the coinmech to start a timer, so you get 10 mins of play for 1 euro for example, no matter how many times you die, that could be a good idea.

you could lock the controls out in the game when the timer expires, then give them a length of time with audio warnings or whatnot to put more coins in and continue, and after a set time, if no further coins are inserted then reset back to the main menu with the buttons locked out.