<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>test</title>
<script type="text/javascript">
function del(){
var str = document.getElementById('text').value;
document.getElementById('text').value = str.substring(0, str.length-1);
}
</script>
</head>
<body>
<input id="text" type="text" value="Какойто текст">
<input type="button" value="Удалить" onclick="del();">
</body>
</html>
Вот как то так.