The best way to delete your old tours is to use TourBuilder's, Import Tool. When you import your old tour into TourBuilder, it will download all of your scenes into TourBuilder. From there, you are able to unpublish the tour from Google.
An alternative to deleting your old tours is through your Contributions on maps.google.com. As long as you own the photos on Maps, you can request to remove them.
This Google Support article walks you through how to delete photos using Google Maps: Add, remove, or share photos in Google Maps.