Cogitate a response that might get under their skin and clarify for them the ugly damage they did. That's not always as straightforward as it seems, as many don't give a f* if they're awful people. It's hard to find what they feel is important and appeal to that vs. a common sense of decency.
If I think they'll go around upsetting other not-particularly-advantaged people (purposelessly), I'll report them. If there's a purpose, I'll try to point out how their method is ineffective.