Regardless of the legitimacy of her claim, the seller cannot harass you and those at your home. You mentioned lawyers, has your lawyer sent her lawyer a letter demanding that she stop harassing you? If so, what response was given?
If a letter from your lawyer does not work, you may need to sue her and seek a restraining order. I would suggest that you use a lawyer who is a litigator; transactional lawyers (like the one you may have used regarding the purchase of the property) can be nice people, but they often don't tend to have the blood-lust that we litigators have.
This is not legal advice and does not create an attorney-client relationship. The information provided is a general statement of the law. Reviewing a case and giving legal advice to a client requires more information than can be exchanged in this format. If you need legal advice, contact an experienced tenants' attorney.
It might be enough to get a restraining order. As the other response advised, if there are attorneys involved already, try to work through them. If that does not work, you could consider calling the police the next time she comes around and report her as a trespasser.
Your last resort should be getting a restraining order since that will be your most costly remedy. They are not always granted, especially in cases that do not involve violence or threats.