The best way to stop someone from doing this is to not give them what they want. The player who is going off by himself wants more attention and has seen that he gets it. The best way to deal with it that I've found? Don't kill him off. In fact, while he's off by himself have nothing at all happen to him. He doesn't find anything, he doesn't encounter anything, nothing at all happens.
Meanwhile, focus on the party as a whole and give them something special while this player is away. Explain that you have to look after the majority of the party, not just him.
Oh - and I agree with the general thoughts that if the bike is the cause of the problems - take it out.
Mike