1 - Block That Shit
Block them. Don't let them respond or comment anymore. You can block them on facebook, twitter, youtube and instagram. This will prevent them from contacting you again unless they sign in under a different name. In which case you can block that name too. If you simply block them and don't respond then it takes away everything from them.
2 - Avoidance
Just ignore them. Don't even respond. If they don't get a rise out of you and you leave them be, they are going to look like a total jackass for making the comment and everyone is going to see how rude they were.
3 - Delete It
You don't have to sit there and re-read that comment and think about the nasty thing they posted. Just delete it. They were out of line and you don't need to have that kind of negativity on your social media. Remove it and the BS is gone.