You have to hide the column in the ExcelMLExportRowCreated
event if you use pre-Q1 2011 version. A possible approach would be to check whether the current item is the last one so that you can hide the desired columns. Dummy code:
(e.Worksheet.Table.Rows.Count == RadGrid1.Items.Count)
Note that this is just an example and not a runnable code - you will have to take the header row into consideration.
the Telerik team