I can easily lay out what will work:
Put mental institutions back in place, give them check and balanced power to intern people that have become dysfunctional.
Dismantle the FBI and start over, because its suspected now that many of these shootings they not only let happen but convince retards to commit these heinous acts (Entrophy, Atomwaffen, to name a pair known to be run by FBI and have kill counts).
Dismantle the CIA and execute the leadership. Because they are responsible for the insane amount of drugs getting into the country and are acting as kingpins to organized crime. None of them should be breathing still for the heinous acts they have committed.
Otherwise? Nope, its going to happen from time to time. Take guns away completley? Mass murders have other easy to access and use tools too :
https://en.wikipedia.org/wiki/Happy_Land_fire