Preventing self-approval of AP Invoices for Independent Contractors

Content

We currently do not allow Contractors to approve invoices, however, we are seeing a need to allow for this when an employee is on leave for an extended period of time. As an independent contractor is set-up as a supplier, is there a systematic way to prevent them from submitting and self-approving their own invoices?  

Example,

Contractor Supplier Name is Consulting Services, Inc. 

Individuals name is John Smith.

John is granted approval authority of $100K in his role as a backfill. 

John submits an invoice for $50K for Consulting Services, Inc. where he is the Requestor. John approves the invoices and is paid.