Supplier receives PO email notification despite unchecking the email Transmission Method when approv

We have a buyer that created a PO, unchecked the email checkbox, and submitted the PO for approval.  The supplier received the PO notification email. 

The supplier has an iSupplier portal account. 

I understand this happened because the supplier has an iSupplier portal account and the contact email associated with the account is receiving notifications. 

However, what are options to prevent such cases?  Suppliers should only receive POs that the buyers have elected to communicate with them.