The fade should be slow and subtle. At first the client thinks they are just imagining it, but then they start getting customer support calls about the site being faded, and their bosses are pointing it out too in meetings, and as it happens more and more the panic really begins to set in.

Finally they reach out to you in a desperation when there’s barely anything left of the site and ask you to urgently fix the problem, and you just shrug your shoulders sympathetically and explain it’s happening because they haven’t paid - but not like in a way that suggests you are doing it on purpose, but a way where it’s simply an unavoidable natural consequence, like if you didn’t pay your electricity bill your power would get cut and the site is slowly “dying” and fading away because of that.

They’d pay so fast.

You automate that shit, you never give them direct access to the source code, and you obfuscate the code that changes the opacity so that it’s really hard to find even if they manage to wrest control away from you. I did this once after the client failed to pay as agreed. They narrowly escaped their site being replaced with a message saying they did not pay their bill, by paying eventually, but I couldn’t let them get away with that shit if they decided to change passwords and tried to screw me completely.