Can I disable the + Section button in the sidebar for non-admins?

Development
,
2

You can use CSS to hide it:

In common css of a theme or theme component:

.sidebar-wrapper .sidebar-footer-wrapper .btn-flat.add-section {
    display: none;
}

If you want it to be available for staff-only, then you can do this:

.sidebar-wrapper .sidebar-footer-wrapper .btn-flat.add-section {
    display: none;
}
.staff .sidebar-wrapper .sidebar-footer-wrapper .btn-flat.add-section {
    display: inline;
}