Go to http://www.livejournal.com/manage/subscriptions/ - there are boxes there to untick for various things. If you've 'tracked' someone's entry - or entries by a particular person (you could, for instance, get notified everytime Zander posts an entry...) - then a box for that appears on that page; when you want to stop receiving notifications, you just untick the box. The relevant FAQ is at http://www.livejournal.com/support/faqbrowse.bml?faqid=272&q=comments