I tried searching and came up with nothing on this topic but I am pretty sure this has been mentioned before.
Why not offer a monthly subscription service for the first year or even a few months? I believe there are people out their that would favor this.
example: Virtualmin 10 can go for $10 a month and after 12 months it is yours to keep.
Most people who are looking for subscriptions don't have all the money to pay at once but can at least afford to pay by month.
I don't know how your key system works, but if it works like other programs where you need to be online in order for the key to function with Virtualmin than all you need to do when someone breaks the monthly contract is render their key useless by blocking it.
I am sure there are other ideas to add on to this.