How do I punish someone who threatened to troll?

Okay this is gonna sound a bit odd, but I had something quite frustrating and at the same time hilarious happen to me.

We were in Champion Select and, of course, as it had to come, someone threatened to feed, troll and afk (in that order) if they didn't get Jhin. He was last pick, so I wasn't gonna take any chances of letting the enemies have the op flavor of the week, and I banned him. He proceeded to go on a 'lengthy' rant about how we're all bad and terrible etc and how he was gonna ruin our game, feed the opponents and basically give them a free win. He also mentioned his thorough belief that, should we report him, it would absolutely not matter since, and I quote, 'this does not work'.

Oh so fortunately, one person, dunno who, forgot to lock in (Imagine my surprise ;^) ) and we were thrown out of CS.

My only regret of not playing with this troll is that now I was unable to actually report him after the game. Assuming he's actually as toxic as he was just in Champ Select, I'd hate for anyone else to play with him.

Is there anything I can do at all to report his attitude even if, honestly, he was only threatening? It's not like I know if he'd actually have trolled given the chance, but considering the banter, I suppose the possibility is very real. All I have to go on are chat logs practically oozing out salt and a dodged ranked game.

(I mean, even if he didn't troll the game, he was already being a very offensive character in chat, and that's reason enough, right?)

If there's nothing to do, I'll have to go on my merry way and let others deal with him, I guess. Just wondering if there's a possibility to prevent his toxicity.