How about telling him politely to stop it? If he doesn't, it is time to stop playing with him. And if it comes to that, you should probably explain why you stop playing with him. That might reduce the negative impact on your friendship/relationship with him/her.
Well, or you can go directly to: "Since I am dragging you down so much, you are better off playing with someone who has better skills than I. I play with those who are just my skill level instead." - that might work, or it might not. It might backfire badly too.
Anyways, I prefer RL solutions to RL relationships. Not technical solution like blocking. That will only cause damage. Social solution has a chance to do little or no harm.