Isn’t there a criminal tresspass complaint that could be filed? Even if this dude actually has a contract (yeah - right), then it’s clearly invalid.
They do have to bring an action for possession of the property (unlawful detainer) to get them out with the help of the law. They can also swear out a complaint for criminal trespass. Self help with force would subject the owners to arrest. It’s a pain but that’s the truth.
This renting of other people’s vacant houses has been big business in Florida for a few opportunistic folks.