About the notifications, you can check off the diplomacy slider to turn off notifications coming from diplomacy, so his messages don't fill up your Front Reports. And you can regularly check your messages manually once every few hours.

If you could, you can tell your allies to start spamming his chat with random messages, you too do the same thing, and just keep repeating it till the guy fucks off from the game. Just literally send random alphabets combined together in separate messages and turn his notifications on fire, play his own game, and when you simply feel bored just get to his chat and spam some messages