Dang,

          If you are speaking of muting a thread via e-mail, I know of no way to do this unless you have access to an individual e-mail message from a thread or a daily digest containing at least one message from a thread.  In either of those cases there is always a link just below the message content itself that allows you to mute that thread by activating it.

          If you are willing to log in to the groups.io site and use the "Your Groups" dropdown at the top of the page and select the "Your Subscriptions" item you are taken to a screen where you have constant access to all messages in all the groups you subscribe to, messages you've muted, messages you've followed and to hashtags that you've muted or hashtags you've followed.  Messages that are muted can be unmuted and vice versa in this page of the web interface.

Brian