No Submit Until Form Is Valid?

I am building a custom validator. I don't want any suggestions on using a jQuery framework, i just want to ask a few question on how best to continue.

Code:
<script type="text/javascript">
//Validator Strings And Expressions.
var Letters = /^[A-Za-z]+$/;

[Code].....

This is the "on page" validation. This, as you can see calls functions held in an included file onkeyup. This all works well, also the username check ajax call works great. However I have 2 questions that I need to sort before this will work correctly.

1. Using M_USERNAME form as an example. If I start typing in there it will begin validating, which is great, However, if i typed in "matt_thomas" based on the validations, Empty:Pass, ValidChars:Fail, Range:Pass, Check:Pass.

So I know that it isn't empty, it's the correct length and it's not already in the DB. But it fails because of the "_". This should mean that I receive the error i have setup for that failure. But I don't. Because I don't stop on a Fail, I receive the success message of the last function.

So my question is; How Can I stop running any more functions when one fails?

2. Looking at the above code, How can I make sure that a submit will only work when ALL fields validate correctly?