Punching damage has been buffed to be pretty high to unarmored people. Punching lightly armored teammates isn't much different from team wounding with your weapon.
Keep your hands to yourself: what might be goofing off to you is ruining the game for others. If someone is slamming M and/or not in your clan or whatever, you are griefing them.
I'll treat this like how I would treat it if I saw it happen though, an hour long ban as a blemish on their ban history and a reminder on what constitutes breaking the rules or just fooling around (this would be against the rules)