Alcoa boss giving bad job ref because pesonality clash?

i want to quit alcoa after 7 years because my vindictive boss is hell bent on trying to get me sacked ive watched him do it to others .upper management let him get away with every thing scott free the harment policies only apply to workers .he will bad mouth me for any job i apply for and i cant blank out the last 7 years of work he has told me that he has done this to previous employees how can i gag this guy legally .i have a family and mortgage like most people and a incurable disease that surfaces when under high stress like some one trying to get you the sack my work out put and quality is high this is a personality clash form a extremely vindictive boss. any ideas