Hide course content from non-members when using the default module of PMPro Courses.

pmpro_courses_show_course_content_to_nonmembers.php

/**
 * Hide course content from non-members.
 * By default, PMPro Courses allows non-members to see the "content" of a course.
 * Lessons are still hidden. By setting this filter to false, non-members will
 * see the restricted view on the single course page.
 */
add_filter( 'pmpro_courses_show_course_content_to_nonmembers', '__return_false' );