NetherBan
Condemn your users to an eternity of suffering!
Downloads Page
Source
Bukkit

Overview:
Basically NetherBan provides an alternative to straight out banning. Banning can be too harsh sometimes and kicking just isn't effective. So I created NetherBan! NetherBan allows you to ban your players and prevents them from escaping. While banished, players can't use portals and have restrictions based upon what you do in the NetherBan.prop file! NetherBan is flexible, it allows for many configuration options and include (but not limited to) mute on ban or can't build and dying will not spawn them in the normal world while banished.

Features:
Ban players to the Nether!
Kick players to the Nether!
Whitelist for your loved ones!
Configurable!
Blacklist items!
Permissions support! (but not required)
Lightning strikes on banish (configurable)

Permissions:
netherban.nbban - Allow /nbban <player>
netherban.nbkick - Allow /nbkick <player>
netherban.nbunban - Allow /nbunban <player>
netherban.whitelist - Allow /nbwl <player> || /nbwhitelist <player>

Commands:
/netherban <player> | Ban a player to the Nether!
/nbunban <player> | Unban a player from the Nether!
/nbwhitelist <player> | Whitelists a player and makes them safe from banishment!
/nbhelp | Displays help menu
/nbversion | Displays NetherBan version you have.