You could send the user_id in the URL string then delete that specific record
http://url.com.script.php?user_id=123
// get the user ID and cast to an integer
$user_id = (int) $_GET['user_id'];
// run the query
$result = $this->db->query("DELETE FROM users WHERE user_id = $user_id");
of course, you'd want to sanitize the $user_id
against SQL injection in your query, just in case...