API to Freeze/Unfreeze a PO

Hello,

We have a requirement, wherein we create a PO, approve it and the PDF is sent to the Supplier.

But we have 2 Suppliers that do not accept UPDATES to a PO.

In such cases, we need to FREEZE the PO as soon as it is Approved, so that users cannot update the PO.

We do have a custom Workflow where, I think, we can call an API (if it exists) to Freeze a PO, based on the Supplier ID.

Can anyone point me to the right direction to achieve this?

Thanks!