When a Steam ID is used, RCON bans should work regardless of whether the player is connected. Currently admins are prevented from easily banning one player from all of their servers simultaneously; they must resort to editing
Bans.json or playing whack-a-mole with an obtrusive user.
This is the RCON response observed when the player is not in the server (Steam ID redacted):
permban 00000000000000000 Could not find player matching "00000000000000000"