What's the most effective yet nonconfrontational way to express your disapproval with someone?

Say it's something mild and opinion based and there's no clical wrong or right way to do this particular thing. For example: Someone who likes to speak the truth rather than mind the feelings of others being disagreed with by a person who is very polite and respectful. And is there ever a time when disagreements are so large that two individuals should disband from the other and cease contact?