If you know the names of the items you can use the "All appointments" view and search. If that does not work you can find the matching "folder-****.json" file that contains the calendar items and remove them manually while SM is not running.

EDIT: To find which folder-***.json to modify you can search inside of "folders.json" to get the "id" of the folder your intending to change, then once you have the id you can find it's file, "folder-<theid>.json"