Word for unusable space

I am describing a problem regarding locations positioned next to each other where blocks are placed. For a given layout the locations next to blocks are unusable and in the next step no block might be placed here. For example, in the layout:

|_|x|x|_|x|_|_|

The first, fourth and sixth locations cannot be used later. I am looking for a word to describe it. So far i have unusable, or maybe dead space. Is there a better word for this? Thanks in advance!

Example sentence: In the layout above there are 3 ___. In another layout there is one __ less.

Things that can be used are available for use. Those that can't are unavailable for use:

not able to be used or obtained; not at someone's disposal

This is a very general term that can be used in a variety of different ways. So, I think it will definitely work in your context.

From Oxford Dictionary (online):

Reserved adj. : 2. kept specially for a particular purpose or person.

• The reserved locations are spaces which may not be used for any other purpose at any time, as they are already serving their purpose.