> My suspicion is that you either can't actually do this, or that the staff have been instructed to make this difficult.

No I imagine they're extremely polite when you try to remove it - that way that get the charge from most people, and the few who actually care about 5% this way or that way will remove it and if you're polite to them you can try to avoid angering them while still getting it from the people who don't care.