Reporting

[Syth`]

Can you report someone for purposely not conceding just to piss the team off?

[Purifying]

Not really. If he still plays and tries to make something happen, you cannot.

[Antimodus]

If he admitted in chat to be doing it just to piss everyone off / teach player X a lesson / "because you don't deserve a concede" and similar then yes you can report him, it's called "griefing".

In other words: it is non-reportable as long as the possibility remains that the player in question did it because he honestly thought there was a chance to win the game, and he continues playing.

If he just keeps playing without saying anything, the doubt remains. Even if it's an absurdly one sided game, it is still up to the judgement of the individual player to make this call. There's quite a number of players with the headstrong attitude that any game can be won as long as the throne is not down, etc. It's considered legitimate.