I disassemble used kegs, clean all the metal parts in oxyclean, and replace all the O-rings. It seems most of the soda smell hides in O-rings, particularly if there's been root beer in the keg.
After replacing your O-rings, take the old ones and put them in your dishwasher with your dishes.
Add 2 cups of TSP (Tri-Sodium-Phosphate)(available at any good hardware store).
Garuanteed cleanest dishes and O-rings you will ever have.....