We sell a software library and get exactly the same thing. When the payment is late enough I email them and say "your license is revoked".

The person/people I'm emailing never ever know that there's absolutely nothing we can do at a technical level to stop the component working. They say "oh my, please please enable it". Well, if you pay, sure. Payment comes as fast as is humanly possible.

In the case of design, my assumption (correct me if you've tried it and it doesn't work) is to retain copyright of the material until full payment is made. Prior to that point, you license them to use the stuff and only assign copyright when you have all the cash.