Why is someone saying, “Don’t feed the troll”?

Feeding a Troll is arguing or giving the Troll material to antagonizes you about. Ignoring the Troll is the only way to make them go away. If people don't respond to their message the Troll isn't going to stick around and move to the next gullible group of goats. Or they will come up with new bait that you need to ignore.