Which version of Excel are you running?
"Backup" means different things in different versions.
Since 2002 debuted there is no incremental backup. Just Autorecovery which
creates a temp file while you're working with a workbook.
If the workbook is closed, whether saved or not, the temp file is deleted.
If Excel crashes while you're working on the workbook, the temp file is saved
and presented the next time you open Excel.
To turn off Autorecovery go to Tools>Options>Save and disable autorecovery.
Prior to 2002 Excel had an AUTOSAVE add-in which made incremental saves to the
actual workbook. It overwrote the file as you worked on it.
If you have AUTOSAVE enabled go to Tools>Add-ins and uncheck it.
Gord Dibben MS Excel MVP