Dennis @ PerfectForms
Create a role for your user and send the notification to that role. Make sure that the state of the submit button is hidden or disabled for that role in the relevant workflow stage. Our help guide covers setting the state of objects based on role and workflow stage: http://www.perfectforms.com/Documentation/manual/html/?fa_setting_a_series_of_states_on_.htm.
Make sure to give your user a way to CLOSE the instance (rather than submitting it). If a user closes a browser tab while an instance is open that instance will be locked until the time-out period has been reached. I like to put a button with a large, bold X in the top right corner of my forms. When pressed it uses a Close behavior object.