Sorry to be sarcastic - but my first thought was "change the locks on her & don't give her a new key!"
Honestly, I would encourage you to seek out an attorney that specializes in property disputes or contract law and ask for a *free* consultation. Many, many attorneys provide a free consultation and will let you know if they think you have a case or not. Take another owner with you so you have at least two opinions on whether or not the lawyer sounds like he/she is a good match for you all. Good luck.