1. hitting it with the back of your racket hand;
2. hitting it with the edge of the bat, instead of the rubber;
3. hitting it with the handle of the bat.
A proviso includes - Your hand is only your racket hand if it is holding the racket, so this means you can't drop your bat and then hit the ball with your hand, because your hand is no longer your racket hand
It is illegal to hit the ball with your free hand