The message is to have a contract and insist on being paid according to the contract, and refuse further work until you get paid.

Have a contract that encodes "fuck you, pay me" into the terms. Ideally, have an actual lawyer take care of the contract and the enforcement. There's a lot of law-y stuff out there that won't hold up in reality. Mortgage companies don't take payment in excuses from your clients, so neither should you.