How about this 100% effective solution: Don't teamhit.
If you do, the say sorry. I sometimes teamhit, and I apologize. If you're forced to type sorry 10 times in 30min, you're showing an incredible lack of caution when swinging.
You don't always have time to say sorry. On horse it's usually a little easier, but if I'm on foot and teamwound I'm generally in the vicinity of a lot of enemies and can't say sorry, same with if I get teamhit on foot, the person is generally very busy and I don't expect them to stop and get killed tryin to say sorry.
It's just assumed that the person is sorry for teamhitting.
I think it would be nice if a message popped up saying you teamhit someone (you don't always know when there's 5 people around you), and having a button to press saying sorry would be nice...but it would be hit every single time, so it's kind of pointless. I think the people who rage hit M are nubs. In war shit happens, you should probably believe that most of the time you get teamhit it's an accident and the person was not being reckless.