This will prevent input of '+' in the textbox:
<!DOCTYPE html>
<html>
<head>
<script>
window.onload = function() {
var keyupHandler = function() {
txtInput = this.value,
lastChar = txtInput.charAt(txtInput.length-1)
if (lastChar=='+'){
this.value = txtInput.substr(0, txtInput.length-1);
}
return;
};
document.getElementById('textbox1').addEventListener('input', keyupHandler, false);
}
</script>
</head>
<body>
<input id="textbox1" />
</body>
</html>