IMPORTANT: This is permanent, no one, not LibGuides administrators, not Springshare, can get you back your guide if you delete it.
This means that boxes that have been reused in other guides will also disappear (if they were mapped, not copied). So, if you have boxes in the guide you are deleting that you know you reused in other guides by mapping to the original (rather than creating a copy), those boxes will also be gone.
With that caveat, sometimes you really do want to get rid of guide. Here's how you do it: