If there is a trip that is no longer active but you do not want to delete it altogether you can deactivate it and specify a reason why. Trips made inactive will be removed from the calendar.
Click on the Inactive button
Select the status reason from the drop down, add additional information (optional) and click "Save"
Status reasons are customizable in Global Settings > Trip > Status Reasons
Helpful Hint: Inactive trips are still accessible by going to Trips on the main navigation, select inactive to true and click search. Make sure to have the correct departure date set or uncheck the departure date filter.