My only other idea is to go on to the game hub/ news tab.
For me there is a button near the top that says "stop following".
If clicking that doesn't work then I'm completely out of ideas. Sorry I couldn't be of any more help.

Quitting the Payday2 steam group should get you off the event notification list, but you'd also lose access to community exclusive gear and heists (if you still play). I'd reconsider the idea of quitting because quite a lot of really good gear and heists are locked to community members only.

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Haha, any time. Hit me up any time, youve got my skype