The trick that was mentioned (sorry, can’t remember by whom) to (1) search calendar items with a single quotation mark (") to find all calendar items, (2) select a range of items you would like to delete, (3) press the DELETE key, and (4) wait for what seems like forever (or start the search over to see that the items are gone), actually worked for me to clean up years of useless calendar entries.
EDIT TO ADD: It was @WayneG! See Easy way to delete entire year of calendar events? - Software - MPU Talk.