MR21 how to restrict.

Hi all,

I am working on a requirment.which is i have list of materials under a plant.i want to authorize someone that he can use mr21 and change the prices of only those materials.but i do't want him to be able to change the prices of all materials that i have at that plant.as i know MR21 is at company code and plant level.so experts please advise me how to make it happen.how can i restrict the use of mr21 from plant level to some level below or anyother suggestion.

THANKS