HN user
Thanks for the info. I have used this method on small scale projects where I provide web hosting, and it worked then. At any rate you're spot-on on having everything in writing before a non-payment should escalate to draconian measures.
I've been in this situation maybe 3x in 10+years, curiously, including my very first web project.
My solution: Turn off their web site. Replace their home page with a message: 'Site down due to non-payment. If you are the owner of this website please contact the administrator immediately' And a link to your own site with your logo on it. (Make the logo BIG! lol)
Never took more than 36hrs to get payment using this method.
Of course this is last-ditch effort, after other options such as asking politely, have been exhausted. And after they pay, I promptly end the business relationship because it's not worth it to do business with customers who drive you to such lengths to get your hard-earned cash.