Only admins can do site bans. What you're seeing is a hacky/temporary feature of the upcoming Lemmy v19.4, of which lemmy.ml is running the pre-release: when an admin bans someone from the site (temp or otherwise), it also automatically bans them from any community they have ever participated in. Lemmy.ml has always been the "beta" instance for new releases.
kuato
joined 2 months ago
https://en.wikipedia.org/wiki/Yaroslav_Hunka_scandal