I always say I am a free man, I can't be limited by anyone. So I have enough courage to say “NO!” to anybody. I don't care who you are and what you do, if I don't agree with you, I will say NO, if I don't want to do what you ask me to do, I will say NO, even if you are my boss.
I have something to say about boss. Boss is nothing. The right my boss has is firing me, the right I have is refusing to do what my boss ask me to do. Under this theory, if my boss orders me to finish a task that I don't like, I will refuse it. What my boss can say is “I will fire you!”, what I will response is “Thank you for firing me, that's what I want”.
Again, I am a FREE man.