Somebody please explain...

[Basil Dreadflint]

I think it's the words you use, more than te idea behind it.

"Buzz off" can deffinately be considered as rude. IT would be much better to take the few minutes and explain to them that what they are doing is stealing your rep, and that you would prefer if they not "help" you unless you ask them to.

If they then continue to steal your rep, then it is they who are being rude.

And, I have been in that exact situation, where I am fighting and someone comes by a "Tags" my enemy just so they can get credit for the kill. A lot of times I'll even invite them to do that so that they can satisfy a quest. But someone who does it all the time, they are not your friend.