unfortunately you can't block people. this happened to me once actually. the way i got him to stop was by whooping his butt a few times. haha. that would be the best option imo.
you could always try just resigning all your games. if it's a tie game or you're ahead then you won't lose a division win. hopefully he'll get the message. also you could try just telling him to stop through the chat though this is risky because if he's a troll then he might start spamming more challenges . another thing you could do is just completely ignore the games. if he doesn't think you play anymore he'll stop challenging you. this would take a while though since it takes a few days to automatically end a game due to inactivity. plus those are games you could be playing against other people since you only get a limited amount of games.
Next time they start a brand new game, don't even bother opening it. If it's anything like last years version, it will disappear on it's own after a couple days, and they won't get a notice that the game is over and they won or lost. I had this happen to me last year, I asked they guy to stop and he flat out ignored it. He did the above to me, naturally in a game I was winning, and he claimed to know nothing about its existence as we played 2 other games. So I pulled the same thing on him, never heard from him again.
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you could always try just resigning all your games. if it's a tie game or you're ahead then you won't lose a division win. hopefully he'll get the message. also you could try just telling him to stop through the chat though this is risky because if he's a troll then he might start spamming more challenges . another thing you could do is just completely ignore the games. if he doesn't think you play anymore he'll stop challenging you. this would take a while though since it takes a few days to automatically end a game due to inactivity. plus those are games you could be playing against other people since you only get a limited amount of games.