Let me show you a number: 15. However interesting, I want to emphasise that this number is mine. You can look at it, but nothing else. No writing it down, no incrementing, no telling a friend about my number, nothing. Of course, I can enforce no such policy. You can write '16' on a piece of paper and hand it to your friend; it's completely out of my control.
An Excel file is just a (collection of) number(s). Probably bigger than fifteen, but the argument is the same. If you control the ink and the paper, you can print any information you can read. As long as you have free space on your hard drive, you can copy all the data you can access.
Depending on the desired level of interactivity, it may be possible to not show the spreadsheet at all. Instead, you could collect the user's input and send results of a server-side calculation back to the client. Spreadsheets are generally not very suitable for this, though, and I know of no simple way to provide the level of interactivity that allows the user can draw his own charts. For that, you may have to hand out the Excel file and be okay with people having the ability to make copies.