It may be possible to get a restraining order as this sounds like harassment to me.
So you can first call the police and indicate that you need their help.
Explain that she comes over and harasses you at every showing.
They may charge her and if they do she will then be placed under a restraining order,.
If they will not they may at least warn her.
As well, you can have a lawyer send her a cease and desist letter threatening a lawsuit for damages for lost potential to sell the property.
If the police will not help you then you can apply for a restraining order on your own.
I don't see what province you are in but in most provinces y7ou can contact the Law Society and use their Lawyer Referral Service. You will be given the name of a lawyer and can consult with the lawyer and the first half hour will be free.