I know two ways of doing so:
1) Delete queue directly from broker. In your case it's Redis. There are two commands that could help you: llen (to find right queue) and del (to delete it).
2) Start celery worker with --purge or --discard options. Here is help:
--purge, --discard Purges all waiting tasks before the daemon is started.
**WARNING**: This is unrecoverable, and the tasks will
be deleted from the messaging server.