Play fighting leads to nothing good IMO but there are instances when a friend or co-worker can't take a hint so here is my 2 cents. Most people hate to be hit, slapped or smacked in highly sensitive areas like the small of the back, ribs or solar plexus. If you have to send a message train your hands to be heavy so you can whack a sensitive area without "aggressive power" into one of these areas. Don't be antagonistic or arrogant about it or invite further conflict