Since you tagged it Javascript, I assume this about not-printing web pages, yes?
If so, then open your HTML file, and just before the closing HEAD tag:
<style type="text/css" media="print">
body {display:none;}
</style>
</HEAD>
It doesn't stop them printing, but all they get is a blank page.
Note: They can still copy an image to the clipboard and print that from a paint program, but AFAIK there is no way to stop that from a web app.