Great responses above. In our house, our dogs down "own" anything, we own everything, and sometimes allow them to have these things, like bones and toys. We control everything. If there is anything like this going on, we put all bones/toys away and give them to them when we decide to, and ask them to sit or something before we give it to them, the "nothing in life is free" method. Then we practice taking the bone back, admiring it, exclaiming what an amazingly wonderful bone it is, pretending to chew on it, even (!) and then giving it back after a sit. This lets them know that you are in charge and it will not occur to them to challenge you for items.
This is also a good time to train "drop it" or "leave it." We use that command for everything from bones, unapproved items (shoes, remotes) or for dangerous things (lighters, poisonous plants or food, etc.). We have even started using drop it when they want to get into it with another dog.
