If you'd like to stop receiving notifications for a single issue, you can unfollow that issue.
You can do this from the issue list by clicking the eye icon to the right of the issue name. Or, if you're viewing an issue, click the unfollow button at the top right.
You can also filter your issue list by following or unfollowing to move more quickly.
Note: If you are the opener or assignee, you'll be prompted to select another team member to be responsible for that issue before you can unfollow it. But after that, you'll stop receiving notifications for that issue unless you comment on it in the future.
If you'd like to turn off all notifications for your user account, just go to Settings, then click the Notifications tab. You can opt-out of email notifications by unchecking all the boxes. If you still want to keep up with the latest activity in your projects, you can choose to receive a daily summary as a less noisy alternative.