I keep a pound-coin-shaped-object clipped to one of the zips on my shopping panniers. It means I have to stop and put the panniers down to un-clip it, but I'm much less likely to accidentally spend it.
It's irritating, but probably necessary in an area infested by drunken students who might otherwise play trolley commandos on the hill. You do occasionally see one parked outside a student house, but you get the impression that it'll be going back on the next shopping trip to reclaim the quid.