How to prevent PO approval in case the approver is the requester in the PO?

We have a Case where the approver of the PO can be the same person who created the Purchase request that the PO was created from.

Is there any possibility that system would prevent such a case that PR requester would approve their own purchases? The PR is automatically approved.

Currently we have the PO Approval rules defined in the BPM worklist. The Prevent self Approval -checkbox is checked in the DocumentApproval task configuration and the Task 'Creator' is selected to the expression builder. This only prevents the case that the PO creator cannot be the same that the approver. Any possibilities to select the Requestor as one attribute there? Any other advice?