codice:
<select name="autore" id="autore" onchange="disableElement('but')">

..............

<button id="but" onclick="ShowHide('show1','newfields1','autore');return(false)">Apri scheda</button>

...............

function disableElement(id){
  if(document.getElementById) {
    el1 = document.getElementById(id);
    el1.disabled = true;
   }
}