There are several ways to unsubscribe. The simplest methods begin at http://arlingtonlist.org/options.htm which has a form where you can enter your email address then press the button marked "Unsubscribe or edit options". (This same form is also available at the bottom of http://arlingtonlist.org/subscribe ).
Pressing the button will bring your to a page where you can either log in with your password (see Getting your password if you need help getting it) to directly unsubscribe, or you can press the button that says "Unsubscribe" on this page. If you use this button, the list will email you a message with a link to click on to confirm that it is you who wants to unsubscribe. This message also comments about replying to the email, but this doesn't always work, as if your email program replies with a formatted message, that formatting can confuse the message processor, thus I suggest using the link.