Cancellation fee for what?

Tried to cancel our alarm monitoring with ADT (it was taken over from National). We have two radios and have changed the alarm system to use only one. Now told I have to pay a "cancellation fee" because i have a 3 year contract with ADT. They did not supply the alarm at all, the alarm was put in by us years ago. The only thing they did was change the radio. So the three contact is for what? ADT prices increase annually so that is not even fixed.