you would need to hand code this using the str_replace() function:
function.str-replace.php
you could use code similar to this on the results page:
<?php
if($_SERVER['REQUEST_METHOD'] == "POST") {
$stripArray = array("and", "if", "or", "the", "at", "as");
if(isset($_POST["keyword"])) $_POST["keyword"] = str_replace($stripArray, "", $_POST["keyword"]);
}
?>
this code assumes a) the forms method is to post and b) the name of the form element is "keyword"