How can I stop this girl from coming around?

Ok, I’m going to keep this semi short because I know most people have lives! Okay so basically, I was dating and engaged to this girl for 3 years, and I say girl because we were both young at the time. We have a 19 month old son, and it was a very hard breakup for the two of us. Mostly for her from my point of view, although she left me. It also turns out I work with people she went to school with, and they are also my good friends. Well, one of these people has a house nearby the two of us, and I tend to go there and drink with the party, as any newly single guy would I think. Well, her younger sister, now 16, seems to make it a point to show up every time I’m there and relay every word I say, while playing the “I’m your friend still!” card. Now here is where it gets bad. This girl and I had a very very s*xual relationship, with some things happening in the bedroom that I wouldn’t want the world to know about. Now she is making it a point to come to this place and make it unenjoyable for me, even though all my friends prefer this spot over any? Any ways I can make her stop coming around without a PPO?