For the moment, with Rental Ninja you can only make rental blockings (unavailable bookings). To know how to do it click here. Therefore, you can only delete rental blockings. In particular, only rental blockings made by you.
To do so, go to that booking (the rental blocking) and while you are on the info tab, you will see a rubbish bin icon. Use it to delete the rental blocking. You will only see the icon if you are eligible to delete it. This means that will only be displayed if it is a blocking you created.