Kick would be used when you intially ban them. Then you can register a function for on_prejoinplayer (
http://dev.minetest.net/minetest.regist ... joinplayer) then you can check the player name against your ban list and if they are banned just return a string in that function stating they are banned and returning the string will deny their access and show them the string you returned. If you don't return a string it lets them in.