In real life, we can quickly tell when people are venting.
So we let them vent. It's therapeutic.
Online, it's more difficult. Are they really angry with us or projecting their anger at a situation on to us? The different is important.
If we push back when anger is projected, we turn a fixable situation into an explosive situation. We make the angry person angrier. Community professionals with a zero tolerance policy do this often. A placatable member can quickly be turned into a lifelong enemy.
When people vent, the recipient's job is to hear them out. It's not to cut them off. It's not to be defensive. It's simply to exhaust every last drop of that anger possible. It's to provide further opportunities to express their anger at the situation.
Don't make false promises ("I'm going to do everything in my power to fix this"). Don't try to win the argument – that's not the objective. Let your ego slide. Very often, it's not about you.
Simply by hearing a member out, asking a few further questions, and perhaps asking how the situation should have been handled differently, you can prevent future trolls.