Don't waste time explaining anything. Here are the 2 best ways to combat trolls:
1: Be ridiculously nice. To everyone both on your team and on their team. The only thing people look at after that is Garen being a ******bag. You will never be looked upon negatively unless the entire game is comprised of trolls. This is countertrolling through kindness.
2: Make up a ludicrously insane argument. While he is having his argument, have your own. If he tells you to stop feeding and that you are a ******/noob, tell him to return your dog or you will alert the authorities. If he persists, keep going. Make it even more audacious and crazy. Describe the dog, state your demands, etc. Eventually his blood will boil. This is countertrolling through insanity.
Or you could just, ignore it and keep playing. If he keeps typing, he's the one that will have more downtime and look like the short bus rider.