I do not believe in karma. I have never seen evidence of it being an actual real thing. Just ignoring them doesn't always help. But here is some food for thought. People who run around throwing insults and being generally rude, hostile and abusive have a problem within themselves not with their chosen target. I take these types on quite often and sometimes win them over or at least defuse their steam. It takes quite a lot in a conversation in the way of patience, but it is always worth it.
I will give you a real example. You may or may not be familiar with a user on this site by the name of Beaver. He showed up to front page memes being a right jerk 100% of the time. He trolled every comment on popular memes. A good friend of mine here and I decided he was going to be our best bud. So we sought him out and made meaningful interaction even when he was being obnoxious. Now, he never really came around 100%, but by treating him as a worthy human when he didn't deserve our consideration we were able to get him to behave just a little better... toward us at the very least. Sometimes if we embrace these types we can nudge them toward better human interactions.
No one behaves in this way without a reason. There is usually something at work behind it. Maybe they feel left out, or jealous, anxious, inferior etc... if I ever run across another one of these here I will find you and direct you to the interaction so you can see my preferred method. You may be inspired to come up with your own way of responding to them next time you get one pestering you. I like your thought process here. You seem generally considerate and caring of others. It never hurts to care for our toxic "friends" as well. Sorry for the long entry here. LOL
Caption this Meme 