If you're using SQL 2008 R2 or later, it's easy to hide elements when the report is exported:
Globals!RenderFormat aka Renderer Dependent Report Layout – Robert Bruckner's Reporting Services & Power View Blog[
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If you're using 2012 or later, you'll also need to check for a render format of "EXCELOPENXML".
To hide an element when you're exporting to Excel, set its "hidden" property to the expression:
=Globals!RenderFormat.IsInteractive = "EXCEL" Or Globals!RenderFormat.IsInteractive = "EXCELOPENXML"
If you're using SQL 2005 or 2008, there doesn't seem to be a solution.