Is there a way to prevent deletion of requisitions by submitter?

[rensims]

We are developing a solution in SAP Ariba Buying & Invoicing that uses the requestion as an intake requests.  Some will never be fully approved and sourced to PO, but they will be actioned on for intake purposes.  We would like to prevent the deletion of these requestions based on the value of a custom field (Intake Status - Completed). 

I tried created an approval node where item would move to that node based on the custom field status = completed and indicating no edits allowed and while that may prevent others from changing, the requestor could always edit and also always withdraw, then delete.  This is what we want to prevent. 

Would love to hear ideas