Have you tried being short with her? 
having been in a similar position to yourself, you can rest assured that she is trying to take control of your life because she has no actual life of meaning of her own. The first thing that you should do is to stop calling her your best friend, because i assure you she isn't. She will be saying the same things about you to others as she's trying to say to you about your boyfriend. You have 2 choices, to try to bore her, but i don't see that happening, or to somehow expose her. Try to record or collect evidence of her lies, because if you say something to her without proof, she'll play victim and use it as further reason to engage and sink her claws into you. Don't expect any reasonable responses or behaviour, no doubt she will be offering free no strings attached blow jobs etc to your boyfriend so make sure you talk to him about it, she's probably told him that you've fucked her on the side.
Basically you need to go cold turkey, rip the band aid right off and cut off contact, she'll squeal and make you responsible for her whole life, but after a short time when she realises you're serious and she's getting no satisfaction, she'll move on to her next target. Do it as soon as you can, don't let her waste another one of your days, because otherwise it'll go on and on and on ...
If you truly want her out of your life but she inserts herself into your social circles then you'd need to isolate yourself and your boyfriend. This isn't a bad idea for a short period of time to see how she would react in front of people who don't see her in the same light. However, it would look controlling if you two are normally very social. Talk to him and try to provide enough proof without your word alone. She could work him by saying you're possessive etc.
Isolating from her own friends? But that means the manipulator's gained that much more leverage over the situation. With a group of friends at the manipulator's disposal, a lot of damage could be done without her there to keep aspects of it in check.
Can't afford to even give an inch, or things just get progressively less favorable.
Ideally the manipulator will show her true colors quickly if her target is the boyfriend. It really depends on what is more tolerable for this person. I had a similar situation when I met an ex and I handled it by trying to just be myself and blah blah. It was much more difficult for me to put on a brave face.
Side note: bitches are crazy.
