Hi,
Today's early morning an idea is jumped in my head. This idea depends on your suggestion about asking the user about printing. However, it has some verifications about his answer.
The verification is supposed to be something like CAPTCHA. It will be some string that does not displayed on screen but just displayed on the printer ( on the printed paper ).
This will depend on DHTML. In other word, in printing page the user will click a button that call function which should do two things:
1- Create a text on the page and using css this is only displayed over printer media.
2- call printer function.
Calling Printer Function leads to blure the window, so using window focus we will call another function that will display none this text, so if the user used print preview he will not be able to read the text.
After that he will be asked to read the text over the printed paper and then insert it in the verification code box to set this page as printed.