I would think about having your DBF answer your phone, and tell her that this is your new number and he thinks you have moved out State, and that you left no forwarding address. Oh and then to what address can he send her the past due bill for the phone service. But then I tend to think of mean things to do to pests.
My observation is that if you give her an inch, she will take a mile or more, and who knows what the BF of the month thinks he can get out of you. I have found that people like this tend to think that you owe them something, just because you used to be sort of relationship. I am very suspicious as to why she now wants to see you right now. There shouldn't be any reason for her to bother coming over, unless she thinks she can get something from you.
I also don't think that she will get the hint that you want to avoid doing anything with her. She may have the intellect of a 12 year old, but that doesn't mean that she understands everything that an average 12 year old should. I think that you need to either tell her, or find away to disappear enough to make it seem real to her. If she could get the hint, she should have done so by now.
The BF of the month is the wild card. I would make sure that she knows that if anything were to happen to you, that she wouldn't get anything, not even a card. Just avoiding her isn't going to work, I don't think she is smart enough to understand the subtly of being ignored. I would let her know that because she doesn't seem to understand your boundaries, that you can no longer keep in touch with her, and that if she persists that you can and will call the police.
You need to play for keeps. Your daughter doesn't need to know this person, just because they shared a possible father. The woman will never be able to be a real sister to your daughter. All this woman can ever be is a burden to your daughter. It will be easier for everyone involved to make the break now, than it will be later on.