One of the most frustrating things about Managed Clients is that the default Dock's items are applied for every user, instead of explicitly using only the items in your list for the managed Dock preferences on the server. I'm amazed that Apple doesn't see this as a problem (assumingly since the haven't fixed it yet). This prevents you from customizing your users' Docks exactly how you want to.
The fix is simple. Open /System -> Library -> CoreServices -> Dock.app -> Contents -> Resources -> English.lproj -> default.plist and delete everything under persistent-apps and persistent-others. I don't delete this file, or the persistent-apps and persistent-others arrays, just to be safe.
If a user has already logged in and gotten received the default Dock items, the easiest fix is to delete their com.apple.dock.plist file. Also, you should delete /Library -> Preferences -> com.apple.dockfixup.plist, as it can also add Dock items.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051012194722981