m0401792, be careful what you wish for! The Canvas Calendar is linked to its course, so if you clear an assignment from the calendar, it will also remove that assignment from the course. Is that what you want to do? If so, the best way to do this is to start fresh. When you import the course into your new course shell, make sure to initiate a selective course copy and deselect all of the assignments and events from the list of available items, so that only the content you want (pages, modules, files, et al.) copies over to the new course. Please refer to How do I select specific content as part of a course import?
So, once they are already in there, there is no way to remove events all at once-clear the calendar?