[jQuery] Username availability check and form validation

[psykeeq]

So I have a JQuery/ajax username availability check function which
calls a file check.php that scans the database  for existing username
and lets you know if the one typed in is occupied or not. And this
availabilty check is inside a registration form witch has some simple
validation rules like required password, required email... The only
think that is getting me frustrated is that even if the uname function
says that the "username is not available" the form lets me through and
I can still press the submit button and go to the next page.
So I want my form not to validate if this function tells that the
username is not available.
I tried to create a custom rule and some other things but I couldnt
get it done so if somebody could help me it would be really great. tnx

$(document).ready(function()
{
        $("#uname").blur(function()
        {
                $("#msgbox").removeClass().addClass('messagebox').text
('Checking...').fadeIn(1000);//remove all the class add the messagebox
classes and start fading
                $.get("check.php",{ uname:$(this).val() } 
,function(data)//check the
username exists or not from ajax
        {
                  if(data=='no') //if username not avaiable
                  {
                        $("#msgbox").fadeTo(200,0.1,function() //start fading 
the
messagebox
                        {
                         $(this).html('This User name Already exists').addClass
('messageboxerror').fadeTo(900,1); //add message and change the class
of the box and start fading
                        });
          }
                  else if(data='yes')
                  {
                        $("#msgbox").fadeTo(200,0.1,function()  //start fading 
the
messagebox
                        {
                          //add message and change the class of the box and 
start fading
                          $(this).html('Username available to 
register').addClass
('messageboxok').fadeTo(900,1);
                        });
                  }
        });
        });