Go to this page: https://boardgamearena.com/gamelist?isWatched=
Subscribed games receive the notification, you should be able to remove it there.
I found your question, when searching trying to add a game to my notifications - it's all very unclear, random games seem to have the bell and there's no way to override it or add to that subscribed list other than going to the game itself, and finding the bell to be there or not in the 'show tables waiting for other players' field...