Also, I don't agree at all with making them feel bad about what they did. That's escalating. I would suggest avoiding this problem (not all problems!) and letting them go away. If they are making you feel bad, why would you want to return that favor? Think about the harm you might be doing to them with retaliation. All kinds of metaphors for this: eye for an eye makes he world blind, violence begets violence.

Plus you'll be living in anger and you'll be exacerbating your discomfort.