It is very hard to stop people that way. You will have to have a license agreement that says they can't do so legally.
If you see all the products in IT industry will work that way. Even VMWare licenses. I can buy a ESXi license and use it on two DC's.
If you still want to go ahead and resolve it, you will have to do an online activation of license. You will give them a key and they will connect to Internet and reach one of your servers to get the key activated. If they do this once, they should not be able to do this again unless they obtain another license/key.