How to report Demotivation?

[DeathCult]

A KotF in one of the recent games I played messed up the game for us. All he did was whine about our picks and complain that we all suck at this game and that we are idiots. After whining to us for some time, he started whining to the opponent team. His continuous whining made us lose our cool and so we lost our motivation to play the game. After a couple of team fights, he disconnected. We tried our best as 4v5 but in vain. Even the opponent team agreed that KotF messed our team play.
I want to know how I can report this guy. I searched RAP, but was not able to find a category that fits his pessimistic attitude that breaks team play. So what category does he exactly fall under? Can any RAP mod please help me?

[MacroHard]

A pessimistic attitude, while frustrating to play with, is not against the Rules of Conduct.

[DeathCult]

Does it mean that I too can be pessimistic and make my team lose and then get away with it?

[SomethingOdd]

I guess, but you'll lose karma.