Aganju: Yes, I know that no matter how good a system is, some people will always try to exploit it. However, until it becomes a large scale problem (and I hope it won't), everything can be solved on an individual basis.
But of course, if someone is given a friendly hint "please do not do it" and refuses to cooperate, his vouchers can be cancelled, passed to other people of my choice, etc. But it would be applied only if everything else fails.