Charge parents for negligence if their child under 21 commits a mass shooting (espeically if it's your gun or it was ordered to your home)
Stronger background checks for guns
Enforce much stronger action to reports of sketchy activity - too many times, other students have warned about these kids, but the schools or police did nothing. Heck, put in tax or financial incentives so they can take it seriously.
Undergo annual counseling/psychiatric evaluation for gun owners. Or heck - make psychiatric/therapeutic appointments standardized the same way that an annual physical check up is done at the hospital.
You don't even have to ban guns entirely. Just hold more people accountable.