In the help file: 2018 SOLIDWORKS Help - Custom Property 

it States:

! = (Not Equal To)

But what do I put in the value field?
It does not seem to be picking up blank, *, or "" when I test it on a file. 

I should note, once a value is entered in the build checks, it can not be deleted. A new line must be created.

Also, why is "not equal to" missing as an option on the drawing tab under "Title Block" of build checks?