Are you trying to prevent everybody from changing the material description or just specific specific users?
All users can be achieved using by changing the material master fields selection. You can create a new fields selection group based on the existing one which contains your material description, move the material description field into this group and then change the settings for transaction MM02 to make the field display only. This would make it impossible to correct mistakes made when creating the material.
If it is user specific then I would say you need a custom authorisation object and an ABAP developer to implement it in a BAdI or user exit.
