Well, this problem could easely been resolved I guess, elevators these days are controlled by microcomputers, shouldn't be hard to add a small program that deselects all selected floors when there is no weight (no passangers) in the elevator. Certainly when all floors are selected in just a few secs and no weight is inside this could be easely detected by the elevator's logic as an obvious act of sabotage.
I think that such a system would either be (a) very frustrating for normal use, (b) very easy to work around or (c) not so trivial in its implementation.
And, if I have to be absolutely honest, I think that the guests you can usually find at the Maritim (i.e. when Eurofurence is not there) are not too keen on that kind of... "elevator humor", so from the hotel's perspective maybe it isn't really too big an issue
EDIT. oh, one thing I forgot.
Do you have € 4,800 in your budget?
Do you need a thing like that? They're not really difficult to build...