How to restrict requesters to approve

Hello all,

I need your help/advice on how I can restrict a requester to not be able to approve, notifications received for purchase requisitions. And for which he/she is not the owner of that purchase requisition.

In our company, the approval rules are based on Job roles, and normally only the users with certain job roles should have the rights to receive and approve the notifications.

But we observed, that some users put a requester in the field Overriding Approves, and the system sent the approving notification, and the requester was able to approve without any issues. And then the approval workflow was generated as per the current rules.