Discuss it w/ the Realtor who listed your property. Sometimes that Realtor has the authority to release you from the listing agreement, and other times, only the broker can do it. For each company, the broker determines the policy on where that authority is vested.
Whether or not I release a listing might depend upon what the reasons are for the request. Unfortunately, there are sellers who will attempt to go around the Broker/Agent to cheat the person/company out of an honestly earned commission. I am not suggesting that is your objective, but wanted to make you aware that agents/brokers must look to this possibility since it does happen.
The cancellation terms are defined in the lisitng contract and can range from as simple as a written request to rare exception.
A shorter listing period of not an answer. Shorter listing periods speak to a weak commitment on your part, and in turn can easily garner a weaker commitment on the part of the broker/agent. An Realtor who is only going to get paid upon selling your property will work harder for you when you make a full commitment to them. In a short term listing, you often find weaker marketing components. Instead of pursuing a short term listing, define the cancellation policy if the Realtor fails to deliver the commitments made. This provides you an out for solid basis while giving you and the Realtor benefit of a stronger working relaitonship.