I had som similar problems with Excel-reports a while back, but my solution was to create a template of the excelfile and in the beginning of the DTS, I deleted the previous report and copied my fresh template.

exec master.dbo.xp_cmdshell 'del excelfile.xls'

exec master.dbo.xp_cmdshell 'copy excelfile_template.xls excelfile.xls'

The great thing with this option is that you can create some basic formatting on the template if your report require that.