What they do not deserve
In many ways (and situations), the best way for us to get our small revenge is to give someone not what they deserve, but just what they don't deserve. If they were mean to us, we treat them kindly. If they were at fault with us, we do not fail them. It might seem silly at a glance, but it actually delivers in two different ways: by showing them (or everyone else) that we are not like them, and by the irony of the situation. And while irony might not win every confrontation, it certainly adds a touch of style.