If you encrypted the entire code, they wouldn't be able to edit anything... so there'd be no reason to give them the source code in the first place ;-)
But, fortunately, there's Custom Tags. Basically you modularize things into separate Custom Tags that each page calls. Then you either encrypt the Custom Tag, or simply restrict access to the Custom Tags directory. Then your editors can edit the code they need to, while your code is safely tucked away in the Custom Tag(s) that they can't get to/see/edit.
-Carl