Although there is no way that I'm aware of to absolutely prevent users from printing a Web page, the following links will provide some ways to make it harder.

http://forums.devshed.com/html-programming-1/how-to-disable-print-40216.html

http://www.felgall.com/jstip35.htm

I just combined the scripts from the two links - and copy and paste the following code. This provides a reminder not to print the page if they right click the page, but if they go to file, print, they get a blank page.

<script>
document.onselectstart=new Function('return false');function ds(e){return false;}function ra(){return true;}document.onmousedown=ds;document.onclick=ra;
function p1(){for(pp=0;pp<document.all.length;pp++){if(document.all[pp].style.visibility!='hidden'){document.all[pp].style.visibility='hidden';document.all[pp].id='ph'}}}function p2(){for (pp=0;pp<document.all.length;pp++){if(document.all[pp].id=='ph')document.all[pp].style.visibility=''}}window.onbeforeprint=p1;window.onafterprint=p2;
</script>

<script language=JavaScript>
<!--

//Disable right mouse click Script
//By Maximus (maximus@nsimail.com) w/ mods by DynamicDrive
//For full source code, visit http://www.dynamicdrive.com

var message="Please do not print this page.";

///////////////////////////////////
function clickIE4(){
if (event.button==2){
alert(message);
return false;
}
}

function clickNS4(e){
if (document.layers||document.getElementById&&!document.all){
if (e.which==2||e.which==3){
alert(message);
return false;
}
}
}

if (document.layers){
document.captureEvents(Event.MOUSEDOWN);
document.onmousedown=clickNS4;
}
else if (document.all&&!document.getElementById){
document.onmousedown=clickIE4;
}

document.oncontextmenu=new Function("alert(message);return false")

// --> 
</script>