Username Availability Check - Stop Submit if Taken

I have a check in place that queries the DB to check if a username is taken.  My problem is that now I need validation on my form field that if taken don't allow the submit, but unsure on how to accomplish.

Here is my code as of now.

HTML

<form class="col-md-12" action="submit.php" method="post">
                        <div class="form-group">
                            <label>Username</label><span id="userResult"></span>
                            <input id="username" class="form-control" type="text" name="username" placeholder="Username" required/>
                        </div>

                        <div class="form-group">
                            <label>First Name</label>
                            <input class="form-control" type="text" name="firstName" placeholder="First Name" required/>
                        </div>

                        <div class="form-group">
                            <label>Last Name</label>
                            <input class="form-control" type="text" name="lastName" placeholder="Last Name" required/>
                        </div>

                        <div class="form-group">
                            <label>Email</label>
                            <input class="form-control" type="email" name="email" placeholder="Email" required/>
                        </div>

                        <input type="hidden" name="submitType" value="6"/>
                        <input type="hidden" name="userID" value="<?php echo $userID;?>"/>
                        <input type="hidden" name="orgID" value="<?php echo $orgID;?>"/>

                        <button class="btn loginBTN">INVITE</button>
                    </form>

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jQuery

<script>
    $("#username").keyup(function (e) { //user types username on inputfiled
        var username = $(this).val(); //get the string typed by user
        $.post('checkUsername.php', {'username':username}, function(data) { //make ajax call to check_username.php
            $("#userResult").html(data); //dump the data received from PHP page
        });
    });
</script>

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