I'm guessing you could delete all other zones (states) from your admin, thus when users register at your shop they won't see the other states.

You could also display some text on the registration page and let people know that you only ship to those 5 states.

1. In /includes/language/YOUR_LANGUAGE/create_account.php, around line 14, change

Code:
define('TEXT_ORIGIN_LOGIN', '<strong class="note">NOTE:</strong> If you already have an account with us, please login at the <a href="&#37;s">login page</a>.');
to

Code:
define('TEXT_ORIGIN_LOGIN', '<strong class="note">NOTE:</strong> If you already have an account with us, please login at the <a href="%s">login page</a>. Please also note that we only ship to WA, OR, ID, MT and AK');
or whatever you like.

2. In /includes/language/YOUR_LANGUAGE/login.php, around line 13, change

Code:
define('HEADING_NEW_CUSTOMER', 'New? Please Provide Your Billing Information');
or around line 16, change

Code:
define('TEXT_NEW_CUSTOMER_INTRODUCTION', 'Create a login profile with <strong>' . STORE_NAME . '</strong> which allows you to shop faster, track the status of your current orders and review your previous orders.');
to include a similar notice.

Always make use of the override function though.

Might not be the best way to do it but hope it helps !

Shirley :)