Some users never learn and don't want to spend the time to figure out how to unsubscribe. As a remote administrator(*), you can unsubscribe them entirely on your own (see Remote Administration). You can also do this from the command line. Alternatively, you can as a normal user send a -unsubscribe request for the subscriber address and let the subscriber confirm. Another alternative is to tell the user to send a message to ‘listname-help@listhost’ and ask the user to follow the instructions, which are very explicit.
Other users have attempted to unsubscribe, but failed. They usually will tell you what they've tried, and forward a list message to you. Most likely, they are subscribed with an address other than their normal sender address. If you're lucky, the subscriber address is in the users E-mail to you. If not, you can ask the user to give you a list of possible addresses. You can then send mail to ‘listname-list@listhost’(*) (if enabled) and use ‘find’ or ‘grep’ to search through the list for parts of the users addresses. If remote subscriber listing is not available, try:
% ezmlm-list DIR | grep -i 'part_of_user_address'
Then send an unsubscribe for the address(es). The user will usually receive a confirmation request (if you found the correct address), and can reply to it to get off the list.
Another possibility is to search the subscription log for a part of the subscribers name, usually shown in the ‘From:’ header (see Remote Listing).