kopyalamayı tamamen önleyemezsiniz ama yine de önlem almak iyidir. Aşağıdaki kod bu konuda yapılmış en iyi kod bence.

<script>

/***********************************************
* Disable Text Selection script- © Dynamic Drive DHTML code library (www.dynamicdrive.com)
* This notice MUST stay intact for legal use
* Visit Dynamic Drive at http://www.dynamicdrive.com/ for full source code
***********************************************/

function disableSelection(target){
if (typeof target.onselectstart!="undefined") //IE route
target.onselectstart=function(){return false}
else if (typeof target.style.MozUserSelect!="undefined") //Firefox route
target.style.MozUserSelect="none"
else //All other route (ie: Opera)
target.function(){return false}
target.style.cursor = "default"
}

//Sample usages
//disableSelection(document.body) //Disable text selection on entire body
//disableSelection(document.getElementById("mydiv")) //Disable text selection on element with id="mydiv"

</script>

<script>
disableSelection(document.body) //disable text selection on entire body of page
</script>


<script>
var somediv=document.getElementById("mydiv")
disableSelection(somediv) //disable text selection within DIV with id="mydiv"
</script>


<script>
var alltables=document.getElementsByTagName("table")
for (var i=0; idisableSelection(alltables[i]) //disable text selection within all tables on the page
</script>

Edit:sağ tıklanarak kaynak kodlara ulaşılmıyordu diye hatırlıyordum ama erişiliyor