You mean a client that hasn't been modified? Then check out Spout and SpoutCraft. You can force only users who are using the SpoutCraft client to connect and everyone else will get a message saying they have to use the SpoutCraft client. AFAIK there are no hacks or mods for the SpoutCraft client.
ex no hacked clients
thanks
http://dev.bukkit.org/server-mods/nocheat
Keep an eye on this: kNoHax
http://dev.bukkit.org/server-mods/knohax
Spout and SpoutCraft
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