Hello,
That API will only delete package metadata from the database, not from disk. It's mostly intended for internal use only, and probably shouldn't be exposed to the API. In any case, we don't store the @ with internally, so if you change @myscope to myscope it should work.
Note that the NPM doesn't provide a way to delete packages, and we never implemented it. There hasn't been any demand for it to date, as people don't really delete packages programmatically - but you're definitely welcome to submit a feature request and help us understand why it'd be a value (like, the workflow you use that requires deleting packages, etc).
Alana