Yes, you can stop your mobile phone contract. However, if you still have time left on your mobile phone contract, and cancel it, most mobile phone operators will charge you a fee. Many operators call this an "early termination fee."
If you are at the end of your mobile phone contract, and do not wish to renew it, simply call your mobile operator and let them know. Get your cancellation in some kind of writing. Some operators may have automatic renewal built into your contract if you do not alert them by a certain date.