I had this problem a while back and the answer was to remove the header from the report. I was going to link the source I got the information from but it is no longer live so I'll do my best with how to go about removing the headers:
You'll have to edit your rsreportserver.config. You can find it under c:\progra~1\Microsoft SQL Server\MSRVersioin_Instance\Reporting Service\ReportServer\rsreportserver.config
Back this file up BEFORE YOU MAKE CHANGES!!!!!!!!!
Open it with a text editor or even visual studio.
Look for <Render>
This will list all the options for exporting your reports.
Add the following code:
<Extension Name="EXCEL (No Header)" Type="Microsoft.ReportingServices.Rendering.ExcelRenderer.ExcelRenderer,Microsoft.ReportingServices.ExcelRendering">
<Name Language="en-AU">Excel (No Header)</Name>
Save your change. Restarting your browser should enable the change, worst case you'll have to restart the ReportServer service.
Hope this helps.