On the left of the Timeline above the calendar, click on the Plan name, and then click the Delete button (trash can icon). This will delete the Plan from the calendar.
Note that this will not remove any Activities from the calendar. If the Plan included Activities when it was applied, and you would now like those Activities removed from the calendar, you may select them individually or as a group to delete them all at once.
Don't hesitate to get in touch using the support chat below with any questions.