That's automatically reported, if you notice the message in chat you get at the end of the game.
How do I report unsportmanlike play?
Yes, I know; however, these seem to be ignored quite often. Most likely, because many of these are connections issues. I've been "reported", because a bad connection caused me to lose the game, not because of I intentionally aborted a game. This guy is purposely making time run out, and I have screenshots to prove it.
I wish Chess.com had a stricter policy against this. Dude's name is CreativeControl. I bet this post will be erased, while nothing's done about him.
okay, thanks for the help, doggystyle. I filled it out, and left this link so the mods can see the screenshots.

okay, thanks for the help, doggystyle. I filled it out, and left this link so the mods can see the screenshots.
That's all you can do.
In live chess, you can right click their name and "block user." I come accross a disappointingly large number of players in live chess (I play 15|10 games) who, upon realizing they're down a piece or about to lose, just disconnect instead of resigning. That's especially annoying when there's 12 minutes left on the clock. I recognize that sometimes connection issues cause players to disconnect due to reasons outside their control, but let's be realistic, if you're 1 move from mating another player and they disconnect, it's probably not because they're having connection problems. So just block the user, let time run out, and you'll never have to play them again.
Hi, I tried looking for how to do this, and can't find it. As of this post, I'm playing someone with 8 minutes left, from who's purposely letting time run out because he's losing (badly). I have screen shots, if proof is needed.