How to be sure that user gets only filtered data in NPrinting 18 and higher releases

A user can get all the data only if no filters are set for that user, this means, no filter at user, task, report or objects level.

1.  A condition can be used to verify that there is at least one filter applied. You can create a Qlik variable that verifies that at least one selection was done on the generated report.
You can use a formula like this:

=If( count(GetCurrentSelections())=0, 0, 1)

the value is 1 if there is at least one selection, 0 otherwise.
I would suggest you to use report conditions in this case, because task, report, and user filters are always applied when evaluating a condition on a report.
See this:

https://help.qlik.com/en-US/nprinting/February2019/Content/NPrinting/ReportsDevelopment/Conditions.htm

2. You can use section access on connections to limit the access to the data. See this article:

 https://help.qlik.com/en-US/nprinting/February2019/Content/NPrinting/DeployingQVNprinting/Section-access.htm

 You can apply section access to NPrinting user by specifying a domain account in their definition (we only accept NTFS authorization).
If you do this, section access limitation will be applied automatically to each users.

 
3. Import users and filters from Excel:

It is possible to import users with filters from Excel. This makes the creation and the maintain of users easier and safer.
You can import the user directly with filters, and once they are created it will be almost really difficult to remove user filters by mistake.
Have a look at this article:

https://help.qlik.com/en-US/nprinting/February2019/Content/NPrinting/DeployingQVNprinting/Import-Users-Filters.htm

 You can use these options in combination to ensure that only correct data is sent to the right person.