There is no current way to temporarily disable the clipboard history.
You can delete a clipboard history element easily enough (eg Delete Past Clipboard: 1) which would keep the items from filling up the clipboard history, but would not avoid them being stored in the first place.
Other than that solutions are more permanent, you can exclude Excel from the clipboard history in the Excluded preferences in Keyboard Maestro, or you can disable the clipboard history altogether, but both of them are semi-permanent.