This shouldn't be a problem, the system seems robust
You sign for every purchase aboard, get a reciept, and get a statement on the last evening of cruise; Anything suspicous
you should ask to see the signed reciept
This should only be an issue if you loose your charge / cabin card, in which case report it straight away so nobody can charge your account
Never had any problems with our onboard account in the 7 time we have been on the island escape just keep all wour reciepts.