If you're not longer using a tracker you can stop billing for the tracking number associated with it by releasing the number. After releasing a the tracking number, the number will not no longer be active but all of the call history and reporting data will remain available.
Go to the Tracker's Settings
To access a tracker's settings, go to your Trackers page and click the manage icon for the tracker you want to update.
Releasing the Tracker
To release the tracker click the scary red button at the top of the page. You'll be prompted to confirm your action as well.