How do you tell a know-it-all to shut up?

very nicely:) other wise you could just say you have something important that you need to do right away and then just leave, but make sure you say it like if you don't do whatever you came up with then you'll be grounded for life or something.

if this person has to be right with everything, then their opinion does not matter to you. If you make this apparent, then this person will be less inclined to give their opinion to you. If you are not friends with this person, or not a person whom they respect then you are more likely to push them to give further reason to speak. Honestly, the best way to deal with this kind of person is to do it in the least offensive manner possible. Take them aside away from your friends and just one on one inform them with out accusing them that they you aren't looking for their opinion and if you want it then you will ask them for it. If you call them stupid because of it, it will cause them to react in hostility to any farther approach you have with them. Then all hope is lost for you in not hearing their know-it-allness.