LAN servers allow bad logins without the password in

When I host a LAN server on a public network, people can join with fake clients that log you in without a password, such as the "Anjocaido launcher"http://gigabytezone.com/load/games/pc/minecraft_launcher_from_anjo_caido/2-1-0-311. LAN servers allow them to log on to your server. It doesn't say the error message "Failed to login: Bad login" like it should be.
This is a very bad issue, since I can't use SMP commands on griefers, because it says "that player is not found", since it's a bad login.
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