How do you do it? Legal issues aside, because I am not a lawyer and cannot give legal advice, the best way is by mutual consent. Do not enter into a contract in the first place if the other party will not mutually agree to a release if requested. You can ask about cancellation policies in the event of a dispute before entering into the contract.
Threatening the other party is rarely a good idea. For example, do not say the agent needs cancel the contract or you will report the agent or write a nasty online review, because that approach is unlikely to gain cooperation and that tactic could also be considered blackmail.