I left a review on someone but since then she has ripped me off so I would like to delete it. It says unapproved anyway so does that mean it hasn’t been posted ?

you could see it when you search for New Reviews for the location you used for the review. If you can’t see it, it isn’t posted.  

Help to warn others away, so they are not ripped off as well.  

If the review status is unapproved it won't be published. click on the review and you'll see the reason that it was not approved.