|DAGS prevent forwarding of email and you'll see hit after hit that explains why you can't stop someone from forwarding an email or at a minimum forwarding the data in the email. You can make it hard, but you can't make it impossible.|
As far as the spreadsheet itself, even if they didn't forward it, they could save it and attach it to another email.
Even if you made it viewable only once by including code to erase all the data once it was closed, they'd still have the original copy that they could download, not open (once they realized what was happening) and sent it to someone else.
They could also take a screen shot of the file and piece it back together so that the data could be viewed even if it wasn't in spreadsheet form.
Bottom line: If you don't trust the recipient to follow your instructions and keep the document private, don't send it to them.
Putting the file on a secure server accessible only by those with randomly changing credentials (e.g. RSA SecurID) can be somewhat safer, assuming you don't allow saving of the data to an external system and assuming - ready for this? - you can prevent the user from sharing his SecurID tag and PIN with the bad guys and/or prevent the bad guy from looking over someone's shoulder while they "legitimately" access the file.
Hey, I got it! Store the file on a single standalone system in a locked room and use the most intense biometric security measures possible - retina scans, finger print scans, voice recognition, etc. If you do all that, your data might be safe from being viewed by those that aren't authorized. Good luck!
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