The watchlist I believe was added after the notification feature, and have common terrain. I’m really not too fluent with tuning them, as I barely use anything other than the default behaviour on them myself. Nevertheless, take a look at the following:

increase your potential with the Watchlist and Notification Feature!   
Watchlist (origins)

If the topic you are trying to remove is not on the watchlist, then it is probably on the Notification configuration tab (https://bitcointalk.org/index.php?action=profile;sa=notification). You can click on an entry to unsubscribe.