Never mind, I figured it out! I turned off cloud syncing, then found the save files (https://support.doublefine.com/knowledgebase/articles/919368--the-cave-pc-save-file-locations) and deleted them all. (Deleting unlocks.sav might have been enough for my purposes, but I wanted to be on the safe side. Also, I initially tried EDITING unlocks.sav to remove all the specific data, which DID clear the cave paintings, but it also corrupted the save file; the game ended up playable but not savable and kept giving me error messages to that effect. Live and learn.)