I got this to work on my site by injecting code into the Header under SETTINGS/ADVANCED/CODE INJECTION. Replace urlslug with the urlslug of the page you wish to hide.
<style>
/* hide page from member area navigation on desktop */
.header-nav-item--folder [href="/urlslug"] {
display: none !important;
}
/* hide page from member area navigation on mobile */
@media (max-width: 640px) { /* or any other mobile breakpoint */
.header-menu--folder-list [href="/urlslug"] {
display: none;
}
}
</style>