You can attempt to prevent alteration of the workbook/worksheets by using
Tools | Protection and using a password to protect individual sheets and the
workbook as a whole.  

But this is absolutely not foolproof.  Excel's 'protection' is not security
- the passwords are easily worked-around or cracked by any number of free or
commercial programs.  And by easily, I mean in a matter of seconds.

Even this 'protection' is not going to stop them from printing, copying,
emailing or otherwise replicating the workbook.

If you need that kind of security and need to show the contents of the
workbook around, probably the best bet is to actually purchase Adobe Acrobat
and turn the Excel workbook into a .pdf file that does permit restrictions on
altering and even printing.  And that's not foolproof.