Restrict or Lock only specific slides

You could delete the "course completion" slide from the Menu. Then users would only be able to access it from the Results slide--and only if they pass the quiz, if the Failure layer disables the Next button and the Success layer enables the Next button.

Here's how to delete an item from the built-in Menu:

• Click the Player button in the HOME ribbon to open the Player Properties.
• Click the Menu button.
• Select the slide you want to delete from the Menu. 
• Click the Trash icon. 

Note: Deleting an item from the Menu doesn't delete the actual slide.

BTW, you can also rearrange items in the Menu. However, that doesn't impact their actual position in the course. The button in the lower right of the above image is the Reset from story button. Use that if you ever need to rebuild the Menu based on the current order of the slides.