It seems there exists no way to remove or disable “Unlock from launcher” in the Quicklist of a Launcher-Icon.
But I found a workaround that saves the current order of the icons in the Launcher and - in the case that an icon was mistakenly removed from the Launcher - restores the order after every start, reboot or login.
Step 1: Save the current order of the icons in the Launcher: Start Terminal (Ctrl+Alt+T) and enter the command
gsettings get com.canonical.Unity.Launcher favorites > starter.icons (this will save the order in the file starter.icons in your home-directory).
Step 2: Create a script in your home-directory by means of the command
gedit ~/starter.icons_restore.sh and type the following text:
gsettings set com.canonical.Unity.Launcher favorites "$(cat starter.icons)"
Save the file and close Gedit.
Step 3: Make the file executable by means of the command
chmod +x ~/starter.icons_restore.sh
Step 4: Make a new entry in the Startup Applications: Klick on the wheel in the top-right corner and chose Startup Applications... > Press Add > Name: Restore Launcher-Icons > Command:
From now on after every start, reboot or login, the order of the icons in the launcher will be restored according the information in the file ~/starter.icons.
P.S: Instead of Step 4 you can also create a Desktop-Starter with the command
PP.S: Good news: In the case you have manually edited the quicklist of a Launcher-Icon, the amendments will be preserved even if the icon is (mistakenly) removed. That means, after restoring, the quicklist still contains all amendments.