Anyone can ruin it deliberately, true - it works best with a cooperative group, not a competitive one. Modifying it for a competitive group would definitely remove it from the real of "useful introductory ideas", but would probably still be a useful exercise for more advanced classes.

Candy is also concrete and engaging - most people don't respond as enthusiastically to raffle tickets or $0.25. As part of a larger set of challenges, using play money with some sort of modest exchange of play money -> small prizes at the end of the session might work.

Doesn't need to be candy, necessarily. Money's the first thing that comes to mind, but if that's prohibitively costly to maintain while keeping the prizes attractive, you could play with probabilities: build a set of diverse prizes of approximately equal value, say, and instead of money or candy distribute tickets to a raffle where the winner'd be able to choose one prize. That might have some funny consequences in this experiment, though, depending on the quirks of how people think about probabilities.