Removing Meta Boxes aka Content Blocks
1) First make sure the display checkbox is unchecked.
Save your changes.
2) The next thing you will want to do is uncheck that meta box from the meta box builder.
3) If you have more than 1 (or multiple) meta boxes of the same type (eg. Showcase 1, Showcase 2, Showcase 3). You can use the Slider under the 'Quantities' tab to remove the meta boxes.
Caution! Be careful here as you still need to uncheck the display checkbox under each meta box before you remove it from the meta box builder or else it will display on the frontend but be hidden from the backend (admin / edit screen).
4a) To remove multiple meta boxes of the same time. Uncheck the display checkboxes, in this case, showcase 2 and showcase 3.
4b) Then slide the quantity slider down to 1.
Note: The slider only works in numerical order, so sliding from 3 down to 1, will remove Showcase 3 and Showcase 2 in that order, regardless of how you have sorted them on the page. That's why we have labelled them 'Showcase 2' and 'Showcase 3'.