How to restrict Supplier/ external user details in Purchase Order reassign user search list

Summary:

While reassigning Purchase Order approval to other Employee, Supplier/ external user details should not come in User name search. 

Content (required):

While reassigning Purchase Order approval to other Employee, Supplier/ external user details should not come in User name search. 

Currently all user details (employee/ Supplier/ external user details) are coming in User name search. So how to restrict this feature so only employee details appear in User search.

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