How do you deal with someone who undermines you?

if I meet someone like that or find one of my friends leaning in the direction of a self righteous know it all like you're describing, I just cut them off---and I don't mean in a conversation but i cut them off of me, out of my life. People like that make me sick and I'm having a very hard time not swearing up a storm because I'm thinking of a few lol. But just stop talking to them they are not worth your time and they will never listen to reason so like I said it's just not worth it. Eventually everyone will leave them and maybe they're realize what a moron they are maybe not but with someone like that I could care less what happens to them. Hope this helps!