You need to do a banishing ritual. Here is a topic about that:
Banishing: What’s the simplest and most effective method?
Also, confidence and authority. Visualize your entire body surrounded with a bubble of white light. Imagine any negative, unwanted energy being absorbed and reflected back. Inside the bubble, there is nothing but calm and peace. Show no fear. Command this spirit to leave you alone and go back where it came! Stake your claim like you really mean it!