I used to be a part of the "Tomb Raider Multiplayer Madness" Steam Group. They have regular weekly events. I became annoyed with the event notifications, so I thought leaving the group would cease them. I left the group. I'm still getting event notifications from the group. How do I stop the event notifications?
In your friends list, there's two tabs - one for friends, another for groups. Click the groups tab, and you'll likely see the tomb raider group there. You can remove yourself from there.
If that doesn't work, go to the Tomb Raider community hub. There should be an unfollow button on the page:
Between these two locations, you're guaranteed to never be notified of any Tomb Raider events ever again.
I found it! As of 2020 Steam. Open Steam, click your Name under the bar:
[Store Library Community Your Name] (Takes you to "activity")
Then on the bottom right you'll see View Groups, and even Upcoming Events including the ones you are sick of seeing.
Click View all X groups.
Find whatever group won't stop spamming you with events and click Leave Group.