If it’s a link or status you’ll need to click on the notification.
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You’ll then see the Unfollow Post link like in this example:
What I’ve noticed though is that you either have to be the person who posted the information or you’ve left a comment on someone else’s post in order to see the Unfollow Post. Oh and don’t worry, if you ever want to reverse the action you can always go back to the post and click Follow Post.
Now since in Facebook several things work together, if you still see notifications for things you’ve unfollowed, check your notification settings.
Hope this little tip helps in cutting down the notifications on those pesky post. Let me know what else you would like to know in the comment section.