How do you deal with those people who act like your friend around you, but they constantly talk shit behind your back.

For example, this guy acts like my friend, we have a class together and enjoy ourselves. However, yesterday he started telling everyone that I said to a girl "I will be your future boyfriend" and as stupid as that is, he made a "meme" out of it. I don't really care about that, I told another friend it was bullshit and that idgaf and that's it. He never says shit like that infront of me though. Infact he even said he would crack some boys skull open, yet he is socially awkward in real life.

But how do you deal with them? Do you cut them out of your life completely, confront them or just ignore it?