Hello,
This seems like a strange requirment and I don't work as much on the AP side so there could be a standard way of doing this, but the question would first be how are you receiving the vendor invoice ? Outside your SAP system and then you manually enter in the details in MIRO?
If so, then on the MIRO screen there is the tax tab where this info sits... In our system, we can not delete this and I'm not sure if thats configuration or a core SAP function.... So if there is nothing standard, then you would need to look for an enhancement to stop the flow once you enter in the PO on the MIRO screen so you can manually make a change to the tax code.