Unfortunately I couldn't find anything that you need to add to prevent the content from being changed. But I have an alternative. To do this, you have to write your own module and just switch this on in the editing area.
I know this post is kinda old but I came across it and figured I'd answer in case anyone else comes looking for the answer. You can use the overrideable=false parameter, so it'll look something like this:
Unfortunately I couldn't find anything that you need to add to prevent the content from being changed. But I have an alternative. To do this, you have to write your own module and just switch this on in the editing area.