The easiest way to leave a list is to click on the link at the bottom of any message from that list; that will take you to the list's home page. Scroll to the bottom of that page and enter your email address in the last form field, then click the "Unsubscribe or edit options" button.
If you don't have access to that link anymore, you may signoff from a UUA email list by going to http://lists.uua.org/mailman/options/listname/emailaddress (replace "listname" with the name of the list and "emailaddress" with the address subscribed to that list) and clicking the "Unsubscribe" button.
A confirmation email will be sent to you. Follow the instructions in that email, and your address will be removed from the list.
Another way to sign off, or unsubscribe, from a list is to send a message with the word "unsubscribe" followed by your list password in the subject line to email@example.com. For example, if your list password were "thoreau" and you wanted to sign off of the UUA-L list, you'd send an email message to UUA-Lfirstname.lastname@example.org with the words "unsubscribe thoreau" in the subject line. If you are unsubscribing an address that's different from the one that you are currently using, send "unsubscribe password address" (where address is the address you are subscribed with).