We have a business requirement to prevent approvers from making any edits on purchase requisitions that are assigned to them for approval. 

There should be a Purchase requisition workflow element titled 'Approve purchase requisitions, editable', and a workflow element titled 'Approve purchase requisitions, Uneditable' (similar to the options available in the purchase order workflow).

If there was a 'Purchase requisitions, uneditable' approval element, this would force approvers to use the 'Reject', 'Request change', etc. elements when the approver requires an edit on the purchase requisition. The edits on the purchase requisition should only be made by the preparer.