Ignore Scott's advice even though he is an agent. If you are talking about a listing contract, all you need to do is speak with the Office Manager or Broker in Charge and explain that you want to cancel the contract. They won't like it because it means they are losing a potential sale but you are the owners and you don't need to sell with this particular brokerage either. What you need to tell the Office Manager is your husband signed the listing contract without your signature. That should be enough to cancel it.
Bottom line - even IF the agent were to find a qualified buyer who submitted an offer, the purchase contract CANNOT be valid because 2 signatures are required - yours and your husband. Therefore, the listing agent will be in the awkward position of explaining to the buyer (or their agent) that he / she took the listing by mistake and the buyer's offer is not valid because YOU will refuse to sign the purchase contract.
Explain this to the Office Manager or Broker in Charge - I guarantee you they will be happy to cancel your listing because NO BROKER ever wants to be in the position of accepting a listing contract that has no chance of getting sold. Brokers need to make money and they only get paid when a home sells - not when a listing contract is signed. So use your power and talk to the Office Manager tomorrow.