YES: some of the best reason for cancelling a listing contract if you are dissatisfied:
1) If you signed a listing agreement and they haven't given you a copy of what you signed, the agreement is considered null & void. If they insist of suing you, tell them you will see them at Real Estate Council (in case of Ontario province, it is the RECO (Real Estate Council of Ontario).
2) You can tell them you no longer wish to continue and you got a refinancing for mortgage.
3) Find a loophole in the contract, like signatures, dates, additional conditions. Just slight mistake will give you a hand in voiding it.
4) if you had a copy of the agreement, send it to a real estate lawyer, they know what to do.