If someone finds that you find offensive something that he/she does and you ask him/her to stop then most of the time they will keep doing it just to force a reaction out of you and by creating this poll... boom reaction.
Social norms and etiquette as you would have it do not apply on the internet, being "polite" in most cases is just a way to get what you want, or someone to do something you want, but that is beside the point here.
If someone is being a prick towards you, respond in kind, or just move on, i doubt this is the first time you have encountered this situation.