There's two things you should do.
One) Do a binding on him so he cannot harm you.
Two) Physically attempt to remove yourself from his/her presence, or seek physical assistance (authorities or help) if it turns violent or is acting very strangely.
Unless if your talking about thoughts in your head, then do a banshing ritual, visualize and meditate on Satans sigil or a demon/ess, and increase your AoP.