I understood this already, but I do not see any sense in it. If the user wants to create a batch managed material then he only needs to select a reference material which is batch managed.
If the user wants to create a material without batch management then he only needs to select a reference material which is not batch managed.
So instead of changing the batch management indicator he only needs to be smart enough to choose the right reference material.
Because of this uncontrolled workaround I do not see any sense in restricting the change of BM indicator.
What is the logic behind this? What was explained to this user how he shall create materials. I mean he must have some guidance weather a material is to be created with or without batch management, isn't it? If he shall not create BM materials, then you can just check if the indicator is active or not. In a real change via MM02 you can always compare the BM indicator from the structure to be saved with the BM indicator from table MARA (or MARC in case of plant specific batch management)