Hello
On your MRP type you can choose the option "External reqmts within replenishment lead time" and then, if the replenishment lead time is zero, you will only consider old reservations.
Otherwise, you can use method CHANGE_MDPSX_MDRS of the mentioned BAdI to make specific reservations not relevant to MRP.
The blog Making an element not relevant to MRP or MD04 using BAdI MD_CHANGE_MRP_DATA provides an example of how to make an element not relevant to MRP.
BR
Caetano