Throwing/trolling games

Just had an experience of a player acting - in my view - unsportsmanlike.
It's in a game, where a player is on his way to soloing, and the rest of us would have to fight him to stop. One players suddenly just makes a nonsensical move away from defending his own centers that the leader is moving forward to try to take. The player instead just moves away and takes a center from another player on our side, and then just surrender no explanation at all.
I don't really know what to do in these situations. It completely destroys a whole game, that you have spent a lot of time on.
It's in a game, where a player is on his way to soloing, and the rest of us would have to fight him to stop. One players suddenly just makes a nonsensical move away from defending his own centers that the leader is moving forward to try to take. The player instead just moves away and takes a center from another player on our side, and then just surrender no explanation at all.
I don't really know what to do in these situations. It completely destroys a whole game, that you have spent a lot of time on.