I try to assume that even the worst trolls could be really real. There are, verifiably, people out there who are that dense*, uninformed*, bigoted*, or just plain guanopsychotic* as to say most any of the things that trolls come here to say. So, I begin with the premise that the absurd thing that I just read was uttered by someone who is legitimately in need of help or gentle correction.
I then set about providing a response, not only to the troll in front of me, but to the wider audience who is reading now and in the future. My conversation may appear to be answering the troll, but the more I doubt their story, the more I am writing for the community, not the OP. It is a challenge, like a game, to take a particularly horrible writing prompt and make something of value out of it.
EDIT: * Please no one assume that any of these adjectives were being leveled at you personally. I'm just calling out some of the characteristics of trolls I've met here. Except guanopsychotic. I love that word and will find every opportunity to use it that I possibly can. So, if you think for a moment it was meant for you, yes, yes it was.