You can try this curl request for your post id
curl -i -sS -X DELETE "http://localhost:4200/posts/<post-id>.json" \
-H "Content-Type: multipart/form-data" \
-H "Api-Key: key" \
-H "Api-Username: username"
To see if it returns a different error but likely it will just say “An Error Occurred”
Looks like there are some site settings around users deleting posts that you can tweak. Also looks like there might be a “Delete all posts” button somewhere.