If, after an interview, you know that you cannot work for a particular judge, then withdraw yourself from consideration immediately, by phone and in writing. Withdrawing before receiving an offer will usually not affect future opportunities. However, if you withdraw your application, it must be due to something you learned in your interview that you could not have determined beforehand (e.g., the judge has his cat in his chambers and you are deathly allergic to cats) or due to an unforeseen change in your personal circumstances (e.g., you need to tend to a sick parent in a different location). It is not a legitimate reason to withdraw your application because, for example, after visiting Dallas (where the judge is located), you decide you could not live there.
Because withdrawal of you application can have serious consequences – not only for you, but also for the Law School and the reputation of your recommenders – you should speak with the judicial clerkship advisor in the Career Services Office before you take any action on withdrawing your application.