Such a function is not built into the SMF forum so we're a bit stuck with what we have.
Might be some mods to install to sort that but I don't find it very likely to take place, especially since there's other diddling going on with the forum/site.
Best you can do is really just to mark all threads as read and just it lay about. As soon as you reply to a thread it will always show up in recent/unread posts and that's just how it works. You can also try to avoid posting in things like game threads where people will constantly post since it will always be active and have new replies.
If you however just meant to unsubscribe from a topic you've asked to be notified on by clicking the "Notify" button inside a thread, you should just go to your profile tab on the forum, then click "Notifications and Email", from there you can choose which threads to unsubscribe from.
