Wanna get rid of a showcase you don't use anymore? Simple.

On a downloaded showcase, you can see on the thumbnail a small grey circle with a line in it. Like this! 

Tapping this prompts a pop up asking if you'd like to delete this showcase. So it's pretty hard to accidentally delete showcases!

Deleting a downloaded showcase will place it back into your list of undownloaded showcases, which you are free to download again by tapping the green cloud icon.