By 1) taking them away when they are arrested and 2) making them harder for criminals to get. That will draw down the number of weapons criminals have.
Well, all the current laws haven't stopped drunk driving, rape or murder either. Still, they are good to have, and they do reduce the incidence of those crimes, even if they are not 100% effective.
The two sentences above do not contradict each other. The vast majority of police are good at a difficult job; in addition police, as a whole, rely on weapons too much.