first and foremost if any one tried to inject themselves between me and my partner, that would be it, I would make absolutly no bones about the fact that she is not welcome and that you do not wish to pursue a freindship with her.
As for other people supporting your decision, not a requirment that they support a descision in order for you to make one and take appropriate action. It is you that is being abused, and you recognize this and want to remove the offending party from your life.
Start by letting her know in no uncertain terms that you do not wish to continue with a freindship with her. If she asks why you can tell her because you are not comfortable with the way it is going, and have made a decision that it is best for you that you just end the friendship.
This is not petty at all, it is abusive, manipulative, selfish and inconsiderate behavior, that could become dangerous if not nipped in the bud quickly. this "freindship" is in reality an abusive relationship.
Have your boyfreind or a real trusted freind present when you tell this person that you no longer want a freindship with her, and want all contact to stop.
Even if it is done by phone try to have someone there. Personaly i would tell her the next time she called me on the phone, for the simple fact that I do nt know if she is willing to go so far as physical assult, but I don't think i really want to risk the possibility that she can and will.
If she continues to phone, text message, show up on your door step, document it, and call the police and let them know what the situation is.
It may be that you may have to go so far as to get a restraining order, depends on how far she is wiling to go with this.
As for the lies, keep documentation of all of the lies and who she said it too, who told you, keep all paper documentation of communications she sends. If it gets to a point were it is beyond ignoring, go see a lawyer or legal advocate and have it brought to court and have slander charges brought against her.
This may sound really harsh, but in reality this person is causing you pain adn trying to harm your relationship with your partner, other freinds, possibly family, and she may be willing to go even further and cause you more harm than she is already doing.
Hopefully you will not have to go to the extreems, and when you tell her firmly and in no uncertain terms that the "freindship" is over and you will not tolerate any inappropriate retaliation IE slandering, stalking via text message or telephone, or email, and do not want any physical contact, she will drop you off her list, and you and your partner can focus on building a happy, healthy life together. Be clear, firm and unwavering. If in the event that she does continue to harras and stalk you get legal help, protect yourself and your rights
She is a malicious person and needs to be out of your life, today!
As for the other people that do not support a healthy decision to get her out of thier lives, tragic for them, I feel really bad for them.
Let me know how things go for you, wishing you and your partner all the very best!
hugs
Tammy