Your browser address bar is not capable of sending a DELETE request. You can use other software such as POSTMAN to make other HTTP requests, including DELETE and POST. it looks like you found the delete instructions, be sure you are using your user api key (not a channel read or write api_key). The user API key is on your profile page.
The proper response for the cleared channel is an empty JSON object, []. The -1 indicates that the server did not complete the action you requested.
