Lock the Dock in Mac OS X
Here’s how to lock down the Mac OS X Dock, all of these commands need to be issued at the command line via the Terminal, so once you’re there enter the appropriate command:
Lock the Dock to Prevent Changes to Contents
defaults write com.apple.Dock contents-immutable -bool yes
Lock the Dock to Prevent Changes in Size
defaults write com.apple.Dock size-immutable -bool yes
Lock the Dock’s Position on the Screen
defaults write com.apple.Dock position-immutable -bool yes
After executing any or all of these commands, you’ll want to relaunch the Dock by killing it:
killall Dock
The Dock will reload itself and the changes will be in effect. Your Dock is now locked!
Why would you want to lock the Mac OS X Dock? If you’re a Systems Administrator or IT Technician, you can make your life a lot easier by having machines stay consistent. One good way to add to consistency across Mac’s is to lock down the Dock to prevent changes in it’s position, keep the contents in place, and to ensure that the size is the same. Now when you’re trying to remotely describe to someone how to open an application, you can be sure it is located exactly where you left it in the Dock.

I would recommend using Secrets – this is a tool with a GUI for hacks like this and a lot of others.
Download Secrets from: http://secrets.blacktree.com/
In the Secrets windows, go to the Dock category and mark the box pointed to in this screenshot: http://drp.ly/mKtdU
[...] Fuente | OS X Daily [...]
How would you unlock?
I have run defaults write com.apple.Dock contents-immutable -bool no (followed by killall Dock) and also defaults write com.apple.Dock contents-immutable -bool false (followed by killall Dock) but to no avail.
Any suggestions?
Try using “FALSE” instead of “NO” and it should work just fine.
Does this lock the dock for all users? Or just the current user? If this is for all users, is there a way to do it just for a single user?
It is only for the user that you are logged in as.
The dock position locking works to remove the options from the System Preferences, however the options from Apple menu (Apple > Dock > Position on Left/Bottom/Right) still work. Is there a way around this so I can completely lock the position?
I wrote a little App LockDock (should work on >=10.4 ?), which uses those commands to lock or unlock the dock.
http://oudkraut.blogspot.com/2012/02/lockdock.html
It asks for admin priveleges first.