You can tap your user icon on the top of the screen and then tap My Subscriptions. Those listed there are the ones you are following and will receive email Notifications. Tapping the "+" next to the topic and choosing "Following" will toggle that subscription off, so you will not get anymore notification emails when a comment has been made.
If you previously chose to follow an entire forum, then under Community to the left of the Topic, you can tap that and if it shows "Following", tapping it again will stop those notifications and will change to just "Follow".
You can also use a more global approach that will stop you from always getting email notifications. After tapping your user icon as before, tap "Edit Profile & Preferences. There you will find a section called Messages from Apple that can be used to control all messages, whether they are from those you participated in or are Following.
And finally when you view a post such as this one, there is a bell icon underneath the initial post on top. If it is filled in, that means you are following the post and will get email notifications. Tapping it will return it to the outline icon and that means you are no longer following it.
