Two things I was thinking just now.
1) It's easier to cure ignorance than to cure false knowledge. Eg. if someone adds up fractions by adding up the denominators and the numerators, he won't listen to you telling him how to do properly. If you're in a teacher-student relation, he'll nod and do it "right" a few times, while (sometimes consciously) thinking "that's how Catbert wants me to do it", and afterwards do it wrong again. You'll have to repeat over and over, correcting every time it's wrong, maybe even a few times showing with pies. My hubby had once a university physics student still adding fractions the wrong way!
2) My policy: never let go a question unnoticed. A question means "I want to know more", letting it go unnoticed means "knowing is not important", which is a message you don't want to convey. If the question is a basic one the student should know already, tell him so. "You should know this already. You'd better look it up now." If the question is an easy one the student should be able to answer, either give a hint as to how to find the answer or let another student answer. If the question is a valid, hard one... answer. Or promise to answer after the lesson, if it would disrupt the lesson. If the question is something you don't know yourself... just say "I don't know." Maybe promise to look it up. A teacher doesn't have to know everything.