When you leave a comment on a hub you will automatically be following that hub you have posted a comment on and will receive notice in your email whenever that hub gets a new comment from someone.
If you do not want to follow that hub (not the hubber, just the hub), and do not want to receive notice every time another comment is posted on that hub, then when you are finished writing your comment and have submitted it, go to the top of the comment section and check the little box (or 'uncheck' if you prefer that description better) that says "following" beside it. When you do that you will notice that the number in that box is one less than before you checked the box. You will still be following that hubber, but you will not receive notices in your email every time a new comment is posted to that hub.
The same thing is true even if you are not following the author of the hub you are commenting on. "Uncheck" the box and note that the number is one less in the box if you do NOT want to be notified when a new comment is posted to that hub. Otherwise you will automatically be notified when a new comment is posted to that hub.
The same thing is true in questions and answers. As I write this answer to your question there is a "follow this question" below this box I'm writing in. If I 'uncheck' that box, I will not be notified in my email when a new answer is posted on this question. If I do nothing with that box I will automatically be notified every time a new answer to this question is posted.