Purchase Order Personalization: stop further processing on Approve button click

I have a requirement to do a personalization in Purchase Order screen to check whether some restricted supplier-item combinations are used in the PO, and to give an error message and stop further processing if the condition is met.

I have done a personalization to give an error message on trigger event- WHEN-NEW-RECORD-INSTANCE, Trigger Object-PO_APPROVE and the error message is triggered as expected. but the user can click 'OK' on the error message and continue with the document approval. How can I stop user from continuing the flow if he gets the error message?

I get the error message as shown below but user can click OK and continue with the flow