<form id="Form1" method="post" runat="server" onSubmit="Form1.Button1.disabled=true;>

Or..

function DoneExam(){

   q = confirm('Are you sure you want to end the tutorial.')

   if (q == true){

     document.getElementById('ucMain_ibDone').disabled = true;

     __doPostBack('ucMain$ibDone',''); return true

    }

   if (q == false){

     return false }

   }