1. Using the Excel export's option dialog
You'll get into this dialog by choosing 'Microsoft Excel Format' in the print options dialog and then clicking on the button on the right hand side of the format combo box. On the 'Output' tab you can choose to export only table data. You can also choose to ignore header and footer lines as well as group lines.
2. Using the API
The parameter Export.OnlyTableData enables you to only export the data from the table object:
Once this property has been set, you can set the group and header line behavior by setting XLS.IgnoreGroupLines and XLS.IgnoreHeaderFooterLines to the required values.
Both ways yield the same result - the Excel file usually will not contain empty cells, which helps very much if the result is to be postprocessed in Excel.