Firstly, because not all My eBay pages have the box that contains the Reviews & Guides link, click My eBay at the top of any eBay page - Now you're looking in the right place.
The link to manage your guides (where you can edit them, choose to display or not display them or delete them permanently) is (BUT SOMETIMES ISN'T!) at the very bottom of the left hand side menu in my ebay. In the Shortcuts box.
If you do not see the link for "Reviews and Guides" in your Shortcuts menu, click the edit button (The button at the bottom, not the link at the top), tick the relevant box and click save. Now you CAN delete your unwanted reviews and guides!