I avoid them. There's no sense in paying attention to them.
Honestly, I feel so sorry for them because the only way they feel content with themselves is when they make other people feel bad about themselves.
When you come across these kind of people, do not return their hate with your hate. It's irrelevant to do so.
Allow karma to do the job. Make it a point to kill them with kindness instead of wasting your time. :)