Even though it would be 99% symbolic, I would support sanctions on gun stores that sell to mass
shooters and associates of the mass shooters.
Gun store owners that sell guns to mass shooters
As well as associates of the shooters... people who didn't report dangerous activities, straw buyers, etc. Travis Reinking, the man who shot up that IHOP, had his father give him back his gun even though he was hospitalized for mental health issues. He would be sanctioned.
Freeze their assets if those are in that country, if a country says "You're banned from entering our country" then I would support that, restrictions on what they can buy like with dual use technology, etc.
Now nine times out of ten, I imagine these people haven't even left their own state in their whole lives, but I think some may be netted and it would send a message that these actions will have consequences.