From within Steam, go to your Workshop page
* mouse over your user name on the top row of tabs
* click on Content on the drop-down menu
* click on the Workshop Items tab
Once on your Workshop page, click on the specific item you want to destroy.
Scroll down below the big picture to the latitude of the text description. Over on the right edge of the screen is a column of things you can do to this item. One of them is a big red button called Delete. Click on that.