The adjective disabused describes a person who no longer has the wrong idea about something. A boy is disabused of his belief in Santa Claus when his older sister tells him it's their parents who fill the stockings.
You'll almost always find the word disabused followed by the word of, and the thing people are most often disabused of is an idea or a belief. A kindergartner might be disabused of the idea that her teacher is always cheerful on a particularly stressful day at school, or a politician could be disabused of his stance against gun control after talking to crime victims. The Latin prefix dis is at the root of disabused, meaning "do the opposite of."