From: http://eventespresso.com/topic/wp-users-allows-editing-others-questions/ and more documentation here: http://developer.eventespresso.com/docs/ee-capability-system-overview/#Core_WP_Wrappers_for_user_check_functions

question_group_caps.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

function ee_filter_edit_question_group( $cap, $question_group_id ) {
    $question_group = EEM_Question_Group::instance()->get_one_by_ID( $question_group_id );
    $current_user = wp_get_current_user();
    $user_id = $current_user->ID;
    if ( $question_group instanceof EE_Question_Group && $question_group->wp_user() != $user_id ) {
        return 'balderdash';
    }
    return $cap;
}
add_filter( 'FHEE__EE_Capabilities__current_user_can__cap__espresso_registration_form_edit_question_group', 'ee_filter_edit_question_group', 10, 2 );