Dew, you are turning it upside down. If I express my attitude by boycotting, it means that I won't be present at the Olympic games. By no means do I dictate the Chinese governement what to do. The only effect it can have, is that Chinese government will miss me at the Olympic games and will start to think about the reason why I didn't come. I don't force them to do anything.
The more people will boycott the Olympic Games, the more will they cause the Chinese government to think about their image in foreign states. But they cannot change the behaviour of the Chinese governent-China is a sovereign state. The only harmful outcome can be that the Olympic games will be without politics, who will thereby demonstrate their attitude to Chinese style of politics. I don't see any reason why sportsmen should boycott the Olympic Games - Chinese do not need sportsmen to come (they have their own sport stars), they need politics, because they want to demonstrate their political power and their position in world's politics.