Only with that agents 'Brokers' permission, or if the agent did not perform theri duties.
If you have a valid listing contract, you are bound to that unless the agent did not fullfill their duties as written in the contract or the broker releases you from that contract. Some agencies will not cancel the contract under any circumstances, except under a court order.
We have a written guarantee that if a client is not happy with our services we will cancel the contract. After all, we provide a service to a client and that service should be top notch at all times.