If you the command found in their FAQ, you will get an error:

*** Access refused: /api/queues/%2F/queue_65e73580-2997-11e5-b513-8b603359ff31/contents

To list all queues:

rabbitmqadmin -H <host> -P <port> -V <vhost/username> -s -u <username> -p <password> list queues name messages messages_unacknowledged idle_since

rabbitmqadmin -H <host> -P <port> -V <vhost/username> -s -u <username> -p <password> purge queue name=<queue_name>