To conceal the "Exit and clear survey" button, use the built-in setting that controls it: in the survey editor, open Presentation in the left icon bar, go to Theme options, and switch off Show 'Clear all' button under Display options.
If your theme does not offer this option, you can instead remove or comment out this line in the theme's nav_bar.twig file (LimeSurvey Cloud users cannot edit theme files):
{{ include('./subviews/navigation/clearall_links.twig') }}
This modification is particularly valuable for long surveys where organizers want respondents to save and resume later. Respondents sometimes click the "Exit and clear survey" button by mistake, which discards their partially completed response.