Do the configuration as
IMG - Plant Maintenance & Customer Service - Maintenance & Service Processing - Maintenance & Service Orders - General Data - Define Account Assignment category & document type for Purchase Requisitions.
In account assignment, keep either Blank value or Some Account Assignment category apart from Order (F).
So, you will not have reference to PM Order while creating PR from PM Order. Result would be, Procurement of materials / services cost will not be booked against that PM Order even though PR has been created from PM Order.
Please provide some justification why you want to create PR from PM Order without imputing PM Order. In that case, you can create Manual PR directly in ME51N.