Wizard not waiting for remote validation response

I am trying to use a remote handler to validate that an email the user is submitting is not already used. This is as part of a wizard using the formtowizard plugin.

However, when i click Next on the wizard it does not wait for the asynchronous response to be returned and goes on to the next step. However, if i go back and wait for the response then it works fine and I cannot continue.

I have tried to force an sychnronous remote request however this is not working.

So the question is, how to make the remote validation check synchronous …

Any help would be much appreciated

jQuery('#SignupForm').validate({
    rules: {
        email: {
            required: true,
            email: true,
            remote: {
                url: my.ajaxurl,
                async: false
            }
        }
    },
    errorPlacement: function (error, element) {
  // Show Error
    },
    errorElement: 'em',
    submitHandler: function (form) {
  // Handle submission
    }
});