If you are working on such a presentation and you have the final version, PowerPoint 2010 now allows you to easily 'mark the presentation as final'. The moment you or your colleagues or friends open this version, they will be intimated that this is the final version and they will be discouraged to make any changes to the file.
How to mark a presentation as final?
Open the presentation. Go to File. Under Info option on the left, go to Permissions and click on the square box named Protect Presentation. Then choose Mark as Final. Save the file and exit. That's it.
If you re-open the file, on the status bar you will find a message; Marked as Final. You can still choose to edit the file, by clicking on Edit Anyway. Unless you click on Edit Anyway you will not be able to make any changes. This way you are ensuring the viewer of the PPT knows this is the final version.