[crazygringo]

This.

You're opening yourself to claims of defamation, tortious interference, disparagement, even coercion, depending on where you are. Not saying the client will win, but they can make it so you'll need to pay lots of legal fees to defend yourself.

It's much smarter to just take the site down without any kind of message, or just something that says "temporarily unavailable". Play dumb with the client, say you don't know why it went down but to fix it but you need to be paid first. Or say it depended on cloud credits that were going to come out of payment, if you don't want it to look like the site went down due to your incompetence.

Making a big public stink might feel good, but it's not a smart business strategy.