Banning anything is never about stopping theoretical "bad actors", it's about reducing the number of normal people who pick up "bad thing" and making mere ownership a suspicious enough act to compel investigation. It's about making things harder. A gun on a black market will be more expensive in a place that makes guns in general illegal. Small time Timmy will not be able to afford a gun to back up his desire to rob the local liquor store.

Systemic design is usually about how you affect the margins