While the instruction to enter "N/A" rather than leave a blank, in the citizenship application, probably is in part to insure document integrity, to minimize the potential for tampered documents, the bigger reason is almost certainly to minimize omissions of material information. This has two important aspects: one is to insure completeness, the other is to insure that material information is included.
In this regard, it is highly likely that the initial completeness screening includes a simple scan for blanks and gaps. Not all blank spaces in the form will trigger a return of the application (to the applicant) for incompleteness. But many will. Returning applications is extra work for CIC, so they want to discourage the instances in which an application will be returned because the applicant failed to enter information. "N/A" indicates, as already suggested, that the applicant has read the item and has made an affirmative decision that the item is not applicable. It is mostly an instruction which requires the applicant to make some response in order to minimize the return of incomplete applications.
Note, however, entering "N/A" will not necessarily work to avoid the return of an incomplete application if, for example, it is an item for which a substantive answer is required.
There is no need to be overly anal about these details. While "N/A" is technically not the appropriate entry when the accurate response is "none," CIC gets it.
Thus, where an item has multiple lines or rows, blank rows following a responsive entry are OK. "N/A" or a strike-through line would be OK as well.
If it makes sense, it will usually work. Best to follow the instructions, to be as responsive to the instructions and particular items as possible, to complete all items in the form, but CIC is not looking for technicalities for the purpose of deeming an application incomplete or for the purpose of denying citizenship. CIC wants a complete and accurate application so that it can efficiently (as efficiently as practical) make an informed decision regarding the application.
On the other hand, there are some key things CIC looks for: every month of the relevant four years, for example, needs to be accounted for in the work history and address history sections. They may not always catch every gap, but that is something they look for and a gap will usually cause a problem.