jquery stop executing after callback function

I have a simple plugin, used for submitting form

(function($){
    $.fn.ajaxSubmit = function(options){
        var defaults = {
            onSubmit:function(){},
        },
        o = $.extend({},defaults, options);

        return this.each(function(){
            $(this).submit(function(e){
                o.onSubmit.call(this);
                $.post(o.url,$(this).serialize() ,
                function(i){"do stuff"},'json');
                return false;
            });
        });
    }
})(jQuery);

And this is how I call it

$('#commentForm').ajaxSubmit({
    onSubmit:function(){
        if($('textarea').val()==''){ "do not execute $.post"}
    }
});

I want to check if textarea is empty, if it is, do not proceed further down the plugin or do not execute the $.post.

Building on gap's answer:

(function($){
$.fn.ajaxSubmit = function(options){
    var defaults = {
        shouldSubmit:function(){ return true; },
    },
    o = $.extend({},defaults, options);

    return this.each(function(){
        $(this).submit(function(e){
            if(o.shouldSubmit.call(this)) {
                $.post(o.url,$(this).serialize(), function(i){"do stuff"},'json');
            }
            return false;
        });
    });
}
})(jQuery);

$('#commentForm').ajaxSubmit({
    onSubmit:function(){
        if($('textarea').val()==''){ return false; }
    }
});

Changed the function name to shouldSubmit as that makes more sense and set the default to return true so it always submits if it hasn't been overridden.

I would have the onSubmit handler return a bool, and let the plugin test that return value.