I use “actually, sorry, but it turns out I'm not invested enough in this conversation to continue, have a nice day/night” and then walk away. I only use it when the discussion is getting heated though, because I'm tired of strangers starting arguments with me about things like morals and human rights (I'm very obviously leftist and live in an ultra-conservative area, so yes it does happen a lot and no that doesn't make me the asshole).