How can I legally lock someone out?

I am sharing an illegal apartment with someone I want to lock out. I am the initial tenant whose landlord put his girlfriend in the apartment to and asked us to share our bathroom and kitchen. After she moved in we found her hostile and harassing. A Court advocate said that since the apartment is illegal we don't have to share our space. How can I make this happen because when I called the police they said we cannot lock her out!