In reply to Gary Jones.

Not every prose results in a binding legal agreement. Even if you do succeed in writing something legal enough that do not contradict with the GPL and is emphasized enough to not fall under false advertisement, you still need to be careful as unfair termination of contract can cause you being sued for damages, therefor in practice you will not be able to be to liberally apply such a clause, and you will have to spend time to investigating and making sure that you are terminating the contract with the right person beyond any doubt, something that is very unlikely you will succeed to do without spending major amount of money.