This can be done! 😀

The way you do this is check the Globals!RenderFormat.Name variable. This will work in 2008 R2 but not earlier versions. Check for the string value EXCEL. You can then hide columns based on this using an expression on the Hidden property:

=IIF(GlobalsRenderFormat.Name = "EXCEL",true,false)

The way you add the hidden expression to an actual column instead of one of or a collection of fields is to click on advanced mode on the right of the grouping pane while a tablix is selected. Your columns will be in the right pane, your rows will be in the left pane. The columns in the right pane will be listed in order top to bottom representing from left to right for the columns in the tablix.

http://technet.microsoft.com/en-us/library/dd255242%28SQL.100%29.aspx

Adjust the "Hidden" property

Frank