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.