How do you deal with toxic clients?

Some clients are true energy vampires. They always complain and are never happy. They don't want to understand technical issues and blame everyone but themselves for every real and imaginable problem. With them, it happens all the time, even if their projects are going very well. When a team leaves a meeting with this kind of client, everyone is exhausted and broken. They really suck out energy and any will to do your best for their projects. How to deal with them? Of course, it's best not to work with such people, but it may happen you already have a signed contract.