Trying to nullify with a Michael Scott-style "I DECLARE NULLIFICATIOOOON!" or otherwise mentioning it is probably a bad idea. I suppose if you'll get warned and you do it again you'll be held in contempt. But I'm no expert.
The situation you describe is known as a hung jury, and would result in a mistrial. Usually this means the case is retried later (with a different jury).
The situation you describe is known as a hung jury, and would result in a mistrial. Usually this means the case is retried later (with a different jury).