Go to the Add/Edit tab and select the listing. On the Modify Listing screen, click on Add Tour/Open House.
Find the tour/open house you want to cancel and do one of the following:
1. Click the Delete button to the right of the event. This will completely remove it as though it was never added.
2. Switch the Tour is Active field to No. This will leave the event in the history of the listing, but make it so it is not visible in Matrix to other users.
Regardless of which way you remove the event, make sure to click Submit Listing when you're finished.