Ajax form validating
Here you can read more about styling of the form and error dialogs.If you want to allow any kind of letters (not only A-Z) you're looking for the letternumeric validator.
See rules() for more details about specifying validation rules.
Use error Class, error Element, wrapper, error Label Container, error Container, show Errors, success, error Placement, highlight, unhighlight, and ignore Title to control how invalid elements and error messages are displayed.
The If wanting to use a custom method that handles the displaying/removing of error messages you should use the two methods described above.
Return false to prevent the default behaviour when adding/removing error messages.
You can however set your own parameter name by using the attribute By using the module sanitize you will be able to declare sanitation functions that will be triggered when inputs looses focus.