This code prevents Sensei learners from starting a course unless they've been specifically added to that course by a site administrator or a teacher.

prevent_registered_learners_from_starting_courses.php

<?php

function sensei_display_start_course_form_when_admin_or_teacher( $should_display_start, $course_id ) {
    global $current_user;

    if ( empty( $current_user ) ) {
        $current_user = wp_get_current_user();
    }

    if ( !( $current_user instanceof WP_User ) || 0 === $current_user->ID ) {
        return false;
    }

    $allowed_roles = array( 'administrator', 'teacher' );
    $user_roles = $current_user->roles;
    $is_administrator_or_teacher = count( array_intersect( $allowed_roles, $user_roles ) ) > 0;

    return $is_administrator_or_teacher ? true : false;
};

add_filter( 'sensei_display_start_course_form', 'sensei_display_start_course_form_when_admin_or_teacher', 10, 2 );