QBO allows you to invite up to 2 Accountants/Bookkeepers into the file without them counting towards the user limit of the subscription. If you need to remove/replace the Accountant or Bookkeeper who has access, you would do the following:
From the Gear Icon in the top right corner, go the Your Company Menu -> Manage Users..
On the Accountants Tab you will see who holds the two slots.
Next to the Accountant Name, under the Action Column, you will have the option to Delete.
PLEASE NOTE: YOU MUST HAVE MASTER ADMIN ACCESS TO THE FILE IN ORDER TO SEE THE DELETE OPTION.