I'd lock the file using CHMOD. Expect some weird behaviour, you will have to remember to unlock it, log-out & log-in before you add or remove items, then log-out, log-in & lock again. (because the dock saves its prefs on log-out)
Doing this will mean they will still be able drag items around, but when they log in again everything will be back as it was.
Let us know if it works!