You didnot tell us where exactly user is changing the Component quantity in the Subcontracting Process. During PO creation or During Goods receipt?
You can not control changing the component quantity during PO creation / Change or in Goods receipt in Standard SAP. You will have to put some control through Badi or User exit's.
If you were telling Bom changes, then you will have to restrict your user to use CS02 transaction through Authorization controls.