Do not allow Paste any non alphanumeric characters

I don’t want user to allow pasting of any non Alphanumeric characters on a text box.

function funAlpha()
{
  var otxt=document.getElementById('txtName'); 

var val=otxt.value;

 for(i=0;i
   {
     var code=val.charCodeAt(i);
     if(!(code>=65 && code<=91) && !(code >=97 && code<=121) && !(code>=48 && code<=57))
         { otxt.value=""; return ; }    

   }

}