If you want to ban guns all together:
1. Make them illegal, offer an amnesty and then impose a prison sentence and/or fine to anyone found to have possession of one, plenty of people will take them up on this because staying out of prison and providing for your family is far more important than having a gun to many Americans, despite what people might think.
2. Stop the sale of ammunition and spare parts, eventually the guns that remain in circulation will become obsolete, stockpiled ammunition will run out and broken guns will not fixable.
3. Wait for cultural change, as gun ownership drops and they are not readily available peoples opinions and views will change, owning a gun will not be normalised and the next generation will not view them in the same light as their parents, the desire to own a fire arm will gradually decline.
All very simple in theory and all would reduce gun ownership, I'm not suggesting that's the approach I would take either but implement those three steps and only an idiot could argue that gun ownership in the US wouldn't be lower in a few decades time.