A forum post asked for a plugin that can ban the name, but not the account. Figured I'd throw one together. Built and tested in 1.16, but probably works in others. I'll test later and update.

Commands

NAMEBLOCK block playername
nameblock.block
Blocks the specified player name from joining

NAMEBLOCK unblock playername
nameblock.unblock
Clears a block on the specified player name

NAMEBLOCK reload
nameblock.reload
Reloads configuration

NAMEBLOCK list
nameblock.list
Shows what names are currently blocked

NAMEBLOCK msg set playername message
nameblock.msg.set
Sets a custom kick message for specified player name

NAMEBLOCK msg clear playername
nameblock.msg.clear
Clears custom kick message from specified player name

NAMEBLOCK msg show playername
nameblock.msg.show
Shows custom kick message for specified player name