Currently you can only delete collections programmatically via the CLI. Otherwise, you have to list and delete every single document in the collection. If those documents have nested collections, you have to (recursively) retrieve and delete every document in the nested collection (As deleting a document does not delete any nested collections the document has). If there are additional levels of nested collections... You get the idea.

I wasn't sure what this other request was referring to so I made this one: https://firebase.uservoice.com/forums/948424-general/suggestions/46562317-api-support-for-deleting-directories