You need to add the following line to your History stored procedure under the Invoice section:
  AND Type <> 'VOID'
Here's my History stored procedure.  Among other modified things, it takes the VOIDs and displays lots of handy info about them so
that you won't get confused about them at all.  I modified it quite a bit several months ago to be much more descriptive than what
came with Emerald.  If you don't want to use what I have below, just add that one line above to the Invoice section and the VOIDs
won't display anymore.
Josh
/****** Object:  Stored Procedure dbo.History    Script Date: 1/29/00 3:43:03 PM ******/
CREATE PROCEDURE History @CustomerID int AS
Select Date, "Invoice", Amount,   CASE
    WHEN Type = 'VOID' THEN 'VOIDed ' + convert(varchar, LastModifyDate)
    ELSE convert(char(10), StartDate, 101) + ' - ' + convert(char(10), EndDate, 101)
  END, InvoiceID