This is sort of an opposite question to keeping quicker students engaged?.
Sometimes I encounter students who are overenthusiastic about the course; they're constantly moving faster than the rest of the class and anticipating material (say, asking a question a minute before I prove a theorem that answers it) or answering ever question I ask the classroom. This is disruptive to the rest of the class, but I don't want to ruin their enjoyment of the class by asking them to not participate.
What's a good solution that keeps such students enthusiastic without allowing them to make learning harder for other students?