Do they have to sign some kind of waiver to play? I would charge people double (or X amount over and above the normal fee) and then refund it after the event assuming they did not spill a drink on it. Similar to renting a condo and putting down a cleaning/security deposit. If they scoff tell them what the machines cost to replace
Would be neat if someone came up with a fluid sensor to install under the lockbar, similar to HVAC when you have a leak in the attic.