You can't turn the turbo "off" per se, since it runs whenever there's enough exhaust to turn the turbines. Mercedes Benz already engineered a fuel saving device into these motors!

If you look on the rear of the intake manifold, you'll see a hard plastic hose. This line goes to the overboost protection switch. When you are decelerating, you still have lots of unused boost from the turbo, and in order to save fuel, the overboost sensor tells the large valve on top of the intake manifold to bleed off the excess boost. The injector pump then realizes that there is not enough boost at this point, so it doesn't inject unnecessary fuel.