If you subscribed to a forum but no longer want to be subscribed to all the boards in that forum, you can unsubscribe to one or more of the boards. Simply go to that board and click unsubscribe. You are not forced to stay subscribed to each board.
How to unsubscribe:
If you unsubscribe to the forum, by default you will be unsubscribed to the boards associated. If you only want to subscribe to one or more boards after you have unsubscribed to the forum, you will need to go to the board(s) and subscribe.