Mouthy 9 year old- suggestions??

I have a 9 year old with a smart mouth too. I've given up on the warnings, explanations, etc. He's 9 - he knows what's expected of him and he knows proper behavior. My new approach? Turn around and walk away. Sounds ridiculous, right? It works. Not ALL the time, but enough to give me a bit of sanity. IE - my kid gets smart I turn around and walk away. Sometimes he tries to engage me, but I do what I need to do until he finally says "Why are you ignorning me?" I simply tell him that I don't talk to people who treat me so poorly. If it's really bad, he loses the next thing he wants, but I don't tell him that until the time comes. IE - he smart mouths and doesn't let up. 3 hours later he wants to watch tv. "Sorry, buddy. I don't reward people who treat me poorly." Life is not perfect. In fact, his attitude can be pretty craptastic at times...but "I" feel better by not engaging with him.