Hi,
As per simple logic yes of course user shouldn't change what has been proposed by MRP run as its on the basis of master data maintained in the system. This again comes from business only. On the other hand what user is changing and why is he/she changing we need to understand. There might me a business/execution logic which we are not seeing as of now but in realty it has a meaning.
After understanding this business meaning with respect to change done we can conclude do we need to alter the master data or do we need to provide extra solution to the business or some functionality.
I hope you would look into why the change is being made and what could be done to avoid this.
Regards
Rahul