You can make your presentation read-only, which means typing, inserting, and other editing features are turned off.
Click File > Info.
Click Protect Presentation, and the click Mark as Final.
A presentation marked as final shows a yellow alert just below the ribbon.
To turn off the read-only protection, in the yellow Marked as final alert, click Edit Anyway.
Marking a file as final is not a security feature – people can undo it and make changes to the presentation. But it tells people that you are sharing a complete version of a presentation.
If you want to truly protect a presentation, set it to be encrypted, or require permission to edit.