I don't know if that is a reportable offense but you can describe it in other ways in the forums and see if any reds agree or disagree:
For example, using the summoner's code,
http://gameinfo.na.leagueoflegends.com/en/game-info/get-started/summoners-code/
Someone who refuses to end a game could be seen as someone who is not supporting their team and actually driving wedges between themselves and the rest of the team (SC1). For better or for worse, in this case worse, the team's fate is intertwined with the Kha's. The game was in full swing and rather than being a positive force for the team, the Kha contributed to the team's demise. Kha was solely enjoying him or herself at everyone else's expenses (SC4) by refusing to end the game and essentially leaving the game by backing until the enemy team was back up again (SC7). Because of this short term afk/leaving, they've started to destroy their relationship with their team (SC5) and openly admitted to intentionally doing it by stating they want to toy with the enemy. This is almost akin to not being humble in victory (SC6). They're trying to gloat and show bad form with this behavior. It's selfish and is destructive in a cooperative team game.