RE: Need to disable PO workflow
Hi Shery,
Setting a workflow as 'Inactive' will not help you to delete that too. But any newly created PO will no more triggered for approval as you have set the workflow inactive.
In case if you want to delete the entire workflow now; then go to Procurement and sourcing > Setup > procurement and sourcing workflows. You might be seeing some instances are still running with that workflow due to which it will not allow you to delete.

You can go to Organization administration > workflow > workflow history. In workflow history form, set the look up as 'Pending' and then filter the record by the workflow ID. You can further filter the record by active version number of that workflow too to get the exact data to act on.

Once this is done, you can simply select all these lines in history and click on 'Recall'. Those pending instances will be recalled in sometime.
Once this is done, you can go back to Procurement and sourcing > setup > procurement and sourcing workflows. Now you will find the instances with that workflow is showing '0'.
Then you can select that workflow and click on 'Delete'. It will be deleted.
Best regards,
Sourav Dam