Ignore them completely. There is no up side in responding to someone who has no sense of dignity or shame.
I block as soon as I see a ridiculous question. If I do get drawn in by a troll and I need an exchange or two to figure that out, I block them rather than try and race them to the bottom in the comment area. It's like trying to reason with a dining room table.
they respond fast so you could see the comment
well if you think about it, it also depends on internet speed but if they're on the same page while you're blocking them, its faster to respond and press "post" so most likely depending on speed, you both block and respond at the same time