!rant but nightly thoughts about wrecking clients that won't pay

If you're building something for a client for money, think of a killswitch.

If it requires internet anyway and you have a webserver, do something similar to what the killswitch of WannaCrypt was (but rather checking for a file on the webserver for existing/non-existing). If the client won't pay, kill the application. If the client pays, maybe even deliver a version without killswitch later (as a "bugfix")

If it is some offline project, you can check for a date (payment due date?) and also check for date/time manipulation on the system itself, and disable the killswitch via a "bugfix" version later just like above.

If clients pay, they don't have trouble. Else, they do.