What if you had in your terms that the website had to utilize your proprietary javascript injected analytics package until the balance was paid in full? That wa

What if you had in your terms that the website had to utilize your proprietary javascript injected analytics package until the balance was paid in full? That way you could inject a "didn't pay" popup similar to NYT when you run out of articles. The client could always edit the code to remove it themselves, of course, but they may not have the skills and if they hire another web developer they'll know they didn't pay the previous one.