As my colleague mentioned earlier, there is no way to achieve this out of the box but what you can do is to apply some hacks on the core file located in /administrator/components/com_easydiscuss/includes/post/post.php and at line 4394 locate the codes below:

[gist]
// Add system notifications for the thread starter
if ($this->post->published && $this->config->get('main_notifications_reply')) {

// Get all users that are subscribed to this post
$model = ED::model('Posts');
$participants = $model->getParticipants($this->post->parent_id);

$question = ED::post($this->post->parent_id);

// Add the thread starter into the list of participants.
$participants[] = $question->user_id;

// Notify all participants
foreach($participants as $participant) {
if ($participant != $this->my->id) {

$notification = ED::table('Notifications');

$notification->bind(array(
'title' => JText::sprintf('COM_EASYDISCUSS_REPLY_DISCUSSION_NOTIFICATION_TITLE', $question->title),
'cid' => $question->id,
'type' => DISCUSS_NOTIFICATIONS_REPLY,
'target' => $participant,
'author' => $this->post->user_id,
'permalink' => 'index.php?option=com_easydiscuss&' . $this->getReplyPermalink(),
'anonymous' => $this->post->anonymous
));
$notification->store();
}
}
}
[/gist]

You could replace it with,

[gist]
// Add system notifications for the thread starter
if ($this->post->published && $this->config->get('main_notifications_reply')) {

$question = ED::post($this->post->parent_id);
$participants = array($question->user_id);

// Notify all participants
foreach($participants as $participant) {
if ($participant != $this->my->id) {

$notification = ED::table('Notifications');

$notification->bind(array(
'title' => JText::sprintf('COM_EASYDISCUSS_REPLY_DISCUSSION_NOTIFICATION_TITLE', $question->title),
'cid' => $question->id,
'type' => DISCUSS_NOTIFICATIONS_REPLY,
'target' => $participant,
'author' => $this->post->user_id,
'permalink' => 'index.php?option=com_easydiscuss&' . $this->getReplyPermalink(),
'anonymous' => $this->post->anonymous
));
$notification->store();
}
}
}
[/gist]

The hack above will ensure that the system only notifies the creator of the main question.