Writing the response somewhere else and realizing the person who you're arguing with isn't having a dialogue with you in good faith, and is purposefully wasting your time. You can't persuade, shame, or prove wrong someone enough to change their behavior on the Internet 90% of the time.
A lot of times I fairly easily ignore stupid replies by remembering that IF I reply I'm just acting like a good little fish and eating up their reply. Instead I just mute em and if I found it to be a particular stupid reply, I send a screenshot to my friend server and make fun of them LOl