If they don't pay you, tell them that they are not allowed to use your software. If they do howeer use your software, they are using pirated software which will

If they don't pay you, tell them that they are not allowed to use your software. If they do howeer use your software, they are using pirated software which will be very very expensive for them (If you call in the police and they confiscate all the computers and data). (Maybe add some function to the code which occasionaly requests a site on your webserver so you can check if they are actually using or not).