A few years ago, I bought a small digital recorder, named it "Bob," and taught the class how to tell Bob their problems instead of me.
I got a beaver (our mascot) stuffed animal and a spiral notebook and had the kids write their problem to the beaver. That pretty much nips it in the bud for a lot of kids (especially ones that hate to write!). You can do the same, or just buy something like this (reallygoodstuff.com).
But, a couple of years ago, I discovered one simple tool to use: a book. Well, that and a discussion. At the beginning of every year, I read the AWESOME book, Don't Squeal Unless It's a Big Deal by Jeanie Franz Ransom. We discuss the sound of a tattle, the 4 instances from the book that you should tell an adult (our school identifies these as "Big Problems") and all of the "Little Problems" that don't need adult help. I give situations and the class identifies each as a "Big" or "Small" problem. This book really has been a lifesaver. In the past, I've done a few different things, but nothing seems to end tattling faster than reading this book (and consistently reminding them "Is that a big or small problem? Did you use an I-message to the person you are upset with before you came to me?").
Or this one, A Bad Case of Tattle Tongue: