So, when you use Decker's main menu bar to got to File > Properties... this thing pops up.
The "Protect..." button here is the specific thing that lets you save a locked deck. Normal saving will save your deck unlocked (with menu bars), because you needed the menu bars to get to the standard save menu option.
If you're running Decker on your computer (it's a little different in Web Decker, but similar enough) the bottom of this "Protect..." save menu looks like this:
That "Save locked deck." note on the screen will be there instead of the usual note if you're in the right place to save a locked version of your project!
(And even though it only says 'deck', it can still be an html file).
Hopefully this clarifies a little bit how it works? But if you're still having trouble please let us know for sure what steps you're taking!