I think that tattling can be good. If someone is doing something wrong, telling an authority figure is a good thing. When children tell us things that others do, that hurt them, the respective child is saying to you:
"I don't have the skills to resolve this problem. Will you help me."
I thank the child for "telling me" because, again, I think it is a good thing. When my kids' are older and one of their friends is not doing the right thing, I want them to tell me.
So I "thank" them and then I see it as an opportunity for discussion. I say to the other child, in my case it is usually his brother: "We don't say those mean things." or I might tell the "offendee" that is getting copied that the "copier" loves him and it his way of playing with them. But then I would say that when someone says "Stop!", it means they aren't having any fun so to respect others boundaries. Respecting others' boundaries is a biggie because the word "No" or "Stop" means exactly that.
So again, I look at tattling as the child's inability to problem solve and that is when I would step in to help give them tools--even if it is like "Tracy" said below, and asking them to work it out on their own. If they cannot, then come and tell me again. Tattling is not a bad thing in my view.
What I find annoying is the whining that may accompany the "tattling" so I tell them to tell me in a normal voice.