1. From 'Dashboard', go to [Settings] [Invoice Approvers].
2. In the [Search for User] search field, type in the name of the User, and [Search].
3. On the left-hand side of the screen, next to [Department], click on the  icon shown in 'blue' as below.
4. On the pop-up window, click on the [Delete Operational / Financial Approver] button, shown in 'red'. This will remove the User from the Approval workflow list for all invoices for that Department.
Note: the Approval workflow of the invoice is different from the Approval workflow for Purchase Requisitions. Removing a User from the Invoice Approvers list will not affect the Purchase Requisitions Approval Workflow.
Please check out this article for further information, Set up Flagged Invoice Approvers.