Here is a solution.
Make a new table of internal account numbers. Set Key field as the Account #. Import your list of 300 Internal accounts.
Make a field on the Internal Accounts list of Type Formula checkbox with a formula of
true
and call it [Internal Account?]
Make a relationships to your data table (1 Internal Account has many data records) and let the system create a field called [Related Account]
Change the [Related Account] field name it Account # mirror and make it a formula field with a formula of
[Account #]
Lookup the field
[Internal Account?]
use it to filter off Internal accounts off your report.
Going forward you can add or delete Internal Accounts using that table.