document.onkeydown =function (e) {  
    var code,type;     
    if (!e){ var e = window.event;}     
    if (e.keyCode){ code = e.keyCode;}  
    else if (e.which){ code = e.which;}  
    type = event.srcElement.type;  
    if ((code == 8)  
      && ((type == "text" && type == "textarea" ) ||  event.srcElement.readOnly == true)) {  
          event.keyCode = 0;  
          event.returnValue = false;  
    }  
   return true;  
};