When you're inside the forum, go to Settings and click on Unsubscribe from this Forum.
How to Unsubscribe from All Forums in a Course
When you're inside a forum, click on Search Forums at the top, right-hand side.
Then click on Advanced Forums in the link breadcrumbs.
Click on Unsubscribe from all Forums. You will be unsubscribed from all forums for which this is an option. Some forums, such as Course Announcements, do not allow you to unsubscribe from them.