They can retaliate, there's simply a cost associated. That's one of the reasons this is a poor exercise: In iterative games it's often worth making sure people

They can retaliate, there's simply a cost associated. That's one of the reasons this is a poor exercise: In iterative games it's often worth making sure people know that certain kinds of behaviour don't work - even where you have to harm yourself in order to do so. Otherwise you'll always be held hostage by those willing to move the costs closest in time. I suspect this is why hatred and resentment are part of our make up.