DD1's prep teacher had a good saying that I have started using when the kids came to her dobbing on someone "Now is that something that I need to know about or is it something you can work through yourself?" Makes them think about what needs to come to me and what doesn't.
I like this too..........

My nieces are terrible dobbers, drives me insane, they even dobbed on ME once

I haven't really noticed DS dobbing yet? What age do kids start doing it?