Racial Slur to a PT Sup

Hey, I’m a new part time sup. Yesterday, an employee with high seniority had an argument with me, I told him to do something and he ignored me for like 10 times. Complete insubordination. Then I got my FT sup and long story short, he called me a “crazy ass n-word”. I straight away went to the shop stewart and he kind of didn’t care. He just said i’ll talk to him. He’s done this before too, being grossly insubordinate and using slurs, i ignored last time he called me the n-word because i was new and thought this would be the norm in the warehouse. What are my options? Also how severe is using a racial slur in that hierarchy of things that can get you fired. I’m at a NorCal hub btw.