Since agents can no longer cancel their own listings, that duty falls upon the Broker, Manager or Administrative staff within their office.
If you are one of these privileged few, here are the instructions for canceling a listing in Matrix.
Go to the Add/Edit menu.
If your firm has more than one office, first select the appropriate office.
Select the appropriate list agent from the drop down list.
That will then give you a list of all of that agent's editable listings. Select the one you need to cancel:
On the Modify Listing screen, click the Change to Cancelled link:
On the Change to Cancelled form, enter the Cancelled Date and click Submit Listing: