ONTRApages uses cookies to store data, so you can either clear the cookies for the page where you have form located, or add a "DELETE COOKIE" button on the page. Here's how to do that second option:
Add a Custom HTML block above the Smart Form block
Add this code to the Custom HTML block:
<center>
<input value="delete cookie" onclick="document.cookie = "contact_id=; expires=Thu, 01 Jan 2000 00:00:00 GMT";" type="button">
</center>
Set your ONTRApage to return to itself by settting the Thank You page to it's own URL.
The process for adding the information will be to first click the "DELETE COOKIE" button, then fill out the form and submit it.