Validate students' comments, but be honest. Some instructors find themselves so excited when students participate that they don't want to discourage them by offering criticism. These instructors have been known to offer comments like, "that's great" when a student misinterprets a text or provides an incorrect answer.
Although it isn't easy to criticize students' comments, it's important to provide such criticism so that other students don't become confused about important concepts. Keep in mind that providing criticism doesn't need to be awful and humiliating. There are tactful ways to give an honest response while still rewarding a student for participating in a discussion.
For example, you could say to the student who gets off track, "That's a really interesting point, but I'm not sure how it directly addresses the question we're looking at. Could you help me understand the connection?" In this case, the student will either offer more explanation, or admit that there was no connection. Using tactful comments allows you to place the responsibility on a student to correct their own response, rather than on yourself.