is account sharing bannable.

I just got out of a rather toxic game. On the enemy team there was a GP who went god mode and pretty much dominated the whole game, he got an early snowball and proceeded to one shot everyone or steal kills with his ult. After the game was done he admitted to boosting his brothers account in the end of game chat. His match history blatantly shows whose playing at the time. Is that not bannable and is there anything i could do to insure the booster is taken care of other then the end of game report.