You could ban the purchase of firearms, but there are so many out there in the hands of the bad guys, that it wouldn't make any difference. The guns themselves are not the problem, it's the people who have access to them.
This is completely untrue.
Gun crime has gone down in every single country in the world that has banned the sale of firearms.
THe idea that there's good guys who should have guns and bad guys who shouldn't have guns would be embarassing coming from a 9 year old. Everyone is a good guy with a gun until they commit a crime, and then it's too late.