"Answer not a fool according to his folly, lest thou also be like unto him. Answer a fool according to his folly, lest he be wise in his own conceit." -Proverbs 26:4-5, KJV

Even though these two verses appear to be a contradiction, there are no contradictions in the Bible--only clarifications. These two verses are actually more like a multiple choice. This passage is essentially "Biblespeak" for "pick your battles." Either you answer him and look like a fool yourself, or you let it go and let him think he is "all that." You get to choose.