Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
<script>
function saludar(){alert('hola');}
setTimeout(function(){saludar=function(){}},1000);//la "deshabilitamos" en un segundo
</script>
</head>
<body>
<form id="form1" name="form1" method="post" action="">
<label for="Submit"></label>
<input type="button" name="Submit" value="Botón" id="Submit" onclick="saludar()" />
</form>
</body>
</html>