Two options here:

1) Save the file as read-only so that the user will need to enter a password to modify the file. Open the file and do a Save As. From the Save As dialog select the Tools button and choose General Options. Enter a password into the Password to Modify field and then complete the save process.

2) Provide the file to users as a PDF so they cannot easily make changes. This is not a 100% guarantee they won't edit as you can open a PDF in PPT and edit if users are aware of this functionality.

Let me know how you go.