Getting rid of old calendar items that waste storage capacity is easy.

Just open the calendar and change ‘View -> Arrange by -> Current View’ to ‘by Category’. Now you got your items listed. Make sure ‘View -> Arrange by -> Show in Groups’ is deactivated. Sort the list ascending by end date. Now you can mark all items by click and either pressing Shift (for all items between the selected two) or Strg (for all clicked items only).
One ‘Del’ does the job.

There ye be wit’ a clean calendar deck. Yo ho!