If this is a data field, use field options to auto-enter "not applicable" into the field. This way the field starts out with that text as the default value.
Put the layout text "Not Applicable" on top of or underneath the field in question. Use a conditional format to hide/reveal the text based on whether the field is empty or not.
You can hide text via a conditional format by either changing the text color to match the background or by specifying a very large text size (such as 200-300 points).