Prevent user to create new elements in a list

I have a list where all the elements are derived from other elements in the model.  I would like the list to behave as follows:

• user cannot remove an element from the list
  
• user cannot add an element to the list
  
• user can edit properties / references / children within the elements of the list.

In short, the list should not be editable, but the elements in the list should be editable.

I know there are issues http://youtrack.jetbrains.com/issue/MPS-18245 and http://youtrack.jetbrains.com/issue/MPS-16865 for stopping users from deleting elements in a list, so this is not possible yet.
However, is it possible to prevent a user to add elements to a list?  In my editors the RETURN will always create a new element.