The search results are stored in a session variable named "$_SESSION["WADbSearch1_Products_Results"]"
you could create a reset link to the results page that contains a querystring variable, ans unset the session variable if that querystring variable is set. for example, create the link:
<a href="results.php?search=reset">Reset the Results</a>
then create the following code at the top of the results page:
<?php
if(!session_id()) session_start();
if(isset($_SESSION["WADbSearch1_Products_Results"]) && isset($_GET['search']) && $_GET['search'] == "reset") {
unset($_SESSION["WADbSearch1_Products_Results"]);
}
?>