You can sue people who do this to you. It's called tortious interference.

It works similarly to people who defame you (slander/libel). Except in this case, it's if they try to break up a contract between you and someone else for a reason unrelated to the contract. Like, it would be one thing if you are a spokesperson and you are fired for saying nasty things. But it's another thing if you are a truck driver and you get fired for saying nasty things away from your employers, coworkers, or clients.