90 year old has mortgage put on house through identity theft, when it was brought to banks attention they found out who did it, bank refuse to prosecute. They eventually wrote the debt off but have not taken their name off her property. What must be done to clear her property.

File suit.

The bank couldn't "prosecute" even if its officers wanted to. Governments, not banks, prosecute violations of criminal law.

Was it a family member who did the fraud ?

The bank needs to provide document that the loan has been satisfied. Not "written off" written off does not even mean the debt is not still owed. In banking terms. Written off is accounting term for tax purpose. She needs a document to show that the loan is either paid in full, or that the loan has been satisfied. That is then used to show there is no lien on home.

*** also, it is not the bank who is doing anything to this women, it is the person who did the ID theft. the bank is merely doing its job.