Código:
<html>
<head>
<script language="javascript">
function apagar(even) {
var evento = window.event || even;
if (String.fromCharCode(evento.charCode) == "a") {
document.getElementById("texto").disabled = true;
}
}
function prender() {
if (document.getElementById("texto").disabled == true) {
document.getElementById("texto").disabled = false;
}
}
window.onload = function() {
document.getElementById("texto").onkeypress = apagar;
document.getElementById("texto").onmouseover = prender;
}
</script>
</head>
<body>
<input type="text" id="texto" >
</body>
</html>

