Help a stranger is taking advantage of my grandmother

So my grandmother is 97 and has what seems to be dementia or Alzheimers. She lives with my 20 year old college student son who keeps an eye on her and her safety when he is home. She started being visited by a women claiming to want to talk to my nana about Jehovah witness publications and that's how she got her foot in the door. She started visiting her often, the next thing we know she starts bringing personal belongings, food and now wants a place to stay. In the beginning she is allowed to stay a few nights a month, but now the lady wants to stay a few nights a week. Money is missing, her i.d. is missing and I asked her to not come back. She refused and says that my nana says it's ok so she is not going anywhere. Ive asked my son to not open the door but my nana will open the door thinking it is a helper who she used to have from before. We tried telling nana that she is a stranger, but nana just gets defensive like a child and makes it about her being able to judge for herself. This women is emotionally trying to scare my nana and says, dont you need me? and you dont want to be alone do you? She if very manipulative. My mother is the owner of the house but she has trouble speaking due to her aphasia from a stroke so I am the one who needs to step up. I work two jobs and have four kids of my own and I love my nana so much but I need advice on how to get this lady away from her.