There are multiple different ways. @Xag_Darklight has a great post on the subject and I have written one on my own technique. Basically, you have to realize that no matter how scary it may appear, you are far more powerful than it. Parasite rely on fear, pain or physical energy in general to exist. If you cut that off (whether by starving it by ignoring its presence or by establishing boundaries through protection or cleansing rituals), they will be destroyed.