Hi Scott
When you enter the material master with the plant and the storage location, on tab MRP4 there is a field called "SLoc MRP indicator". If you set the value "1 Storage location stock excluded from MRP" for this field, the quantity under this storage location will not be considered by MRP.
You can also control which users are authorized to post goods movements for this storage location using the authorization object M_MSEG_LGO.
BR
Caetano