Fencer: First of all, thanksfor putting the offer up!

Question is, as you are stating that the vouchers cannot be used for the buyer himself, you obviously don't want people to use that as a simple 'buy 1 year, get 2.5 years', but for *giving* to others - very nice also in the sense of Christmas. But now John could buy a year and give his vouchers to Jane, and Jane buys a year and gives her vouchers to John, basically circumventing the limit. Do you actively discourage that, meaning you will not assign the vouchers then? Or is that accepted?
I thought about not posting that to not give people ideas, but, realistically, many will come up with this. Easily.

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.