function my_stuff() {

    if ( isset( $_GET['deactivate'] ) ) {

        $user_id = wpf_get_user_id( $_GET['contact_id'] );

        // Set user to default usergroup

        WPF()->member->set_usergroup( $user_id, get_option( 'wpforo_default_groupid' ) );

        // Remove from LD group #123

        ld_update_group_access( $user_id, $group_id = 123, $remove = true );

        // Remove from course #456

        ld_update_course_access( $user_id, $course_id = 456, $remove = true );

    }

}

add_action( 'init', 'my_stuff' );