Control for Purchasing (Inhouse/External) is at Material Type level. MRP Tab in material master is used only for Controlling Procurement Proposal creation during MRP. You can create PO without MRP view in material master.
If you want to block purchasing for particular material Define Material Status Key in customizing.
Define a Plant-Specific Material Status and Select "E-Error" in Purchasing and assign to material master/Plant.
Regards,
Niranjan