masukomi, 
TIL that (on mastodon) if you make a post "Followers only" it ALSO means your followers can't boost it.
This is awesome,
BUT there's more. IF You reply to someone who DOES NOT follow you AND you set the reply to "Followers only" THEY can't boost it either.
Of course, there's no getting around malicious screenshotting, but EVERY bit of control we give users over the way their content is shared is good.