I was sooo tempted to do this with a client who owed me $30K

But I was worried about getting sued by that client.

Yes. What he did is generally not good advice / a good course of action.

What you do is let them know you expect payment in X days, and if not start legal proceedings. And then you follow up on that.

Meh. I wasn’t going to bother getting a lawyer involved for a balance of a few thousand dollars. I did threaten legal action though it proved futile.

Out of curiosity, why was it not a good course of action? Did the client have any leverage after being 6+ months past due on their payment?

Lol, with all respect, taking down a companies website even with credentials given to you (for another job) can be construed as hacking. Revenge hacking can get ugly pretty quickly (criminal offence). You have a contract for X, do X and don't get paid for X. The conflict over paid for X is in a whole different ballpark than illegally using credentials for Y (taking down their site, with obvious damages as a result).

I was more referring to the $30k example above. In your situation probably fine for a few thousand, but if someone owes you a significant amount of money then I don't think you should act in this way. You just sue them (if all else fails).

Netflix is providing a service. It's on their computer. They are certainly entitled to terminate your access to it. Logging into someone else's AWS account and deleting their stuff is, in the eyes of the law, more akin to Netflix breaking into your house and stealing your TV to stop you watching Netflix when your credit card payment is declined. Or remotely formatting all your computers to make sure you can no longer access your Netflix account. That is not the correct legal path to remedy the problem....it's a felony. It's all well and good to play hardball with people who are not paying you, but the correct way to do that is with lawyers. If you commit a felony first, then they can tell you to go pound sand and your choices are now (a) pound sand or (b) Get your 2 grand or whatever was owed, pay half of it the the lawyer, and then go to jail when they report you to the police out of spite.

Here is the real question...

Can you program in some code to display on the website “this website owner needs to pay for their site” etc.

and if they don’t pay you, you don’t disable it. And it activates after a while. Plus it would have redundant things so they can’t easily just hire one guy to remove it.

Or it could be some thing that breaks the site in a less obvious way and they call you to fix it.

It doesn’t seem to break any laws that I know of. Do you?

I am not a lawyer, and your scenario is getting into the gray zone where you really ought to consult a lawyer to get an even remotely reliable opinion. That said, my understanding is that: You shouldn't get convicted(no crime was committed if you do it right). You might still get arrested(have fun convincing officer donut that you remotely disabled some business's website without hacking). You might get sued in civil court for damages. You may or may not win the civil case depending on the wording of your contract, implementation details of your sabotage, the mood of the judge, and the alignment of the stars. I would still argue that a few hundred bucks to hire a lawyer to send a form letter is safer, easier, and more reliable, assuming of course that the client resides in a country with reasonably strong rule of law. You're trying to use a hacker solution to a lawyer problem, which is both unprofessional and generally frowned on by the legal system.