If you start or participate in a topic, the forum will automatically send you an e-mail alert when there is new activity in that topic. This can be a good thing, since you don't have to keep coming back and checking for replies to a topic you have started. It's also a good thing because later (sometimes much later), another member may post to ask you a question, or to answer a question you have asked.
We also have members who post about a certain situation they're in, and we ask them to come back and let us know how everything turned out, which helps the next person who later finds themselves in the same situation. I think it's a small thing to ask considering how much this community helps everyone.
But if you want to opt-out of following a topic, you can do so. While posting a reply in the topic, while viewing the topic, and in your personal notifications settings.
Option 1: While Posting (Just Before You Save Your Post)
If you start or reply to a topic, by default, you'll receive an e-mail alert of any subsequent posts in that topic. Before you save your topic-opener or reply, you can disable the automatic alerts by toggling the "Notify me of replies" option.
Option 2: While Viewing a Topic
You can also turn off the reply notifications while viewing the topic. At the top of every topic there is a button which tells you if you are following or not following the topic. If it says "Following" and you want to "un-follow" it, just click that button.
That will open up a menu of "follow" options. If you just want to un-follow the topic, click the "Unfollow" button that appears.
Option 3: In Personal Notifications Settings (This Turns Off ALL Notifications)
If you want to stop following ALL topics, click on the "bell" icon at the top right of the forums. This will open your notifications.
Click on "Notification Options."
Now un-click the follow options as depicted below: