The Next/Previous Quiz links come from the theme so they can be disabled only with the custom CSS code. As every theme uses a different set of CSS classes, so I need the URL of your quiz so that I can provide you the CSS code.
And QSM does have an option called “Should the user be required to be logged in to take this quiz?”. If this setting is set to YES, then only the logged in users can take this quiz.
then I think you can install a plugin that only you can see that information and no one else can see it, you should try it tiny fishing I think this is the right plugin for you
To remove the previous/next quiz links at the bottom of each quiz page, you can use custom CSS. These links are typically generated by the block blast, so hiding them requires a bit of CSS tweaking.