Bukkit

7.5 Update!

• Players are now protected from PvP inside their land claims by default (exception: siege mode servers). You may disable this in your config file.

There's a bug causing a "my brother got banned when I did" problem:

https://bukkit.atlassian.net/browse/BUKKIT-3521

There is no workaround, except to disable smart ban entirely, which will open you to attack by griefers who are aware of the vast lists of username/pass combinations published on griefer forums and websites everywhere.  So, only bad choices here.  Note that if you /pardon all affected players, they will all be able to log in again (meaning you have to /pardon the bad player along with the good ones).