Sometimes people forget to follow the best list etiquette guidelines, and when replying to a digest, their message quotes the entire digest instead of only the relevant portions of a particular message.

Reminding your users periodically to trim their replies and adjust their subject lines to something relevant is definitely a good practice. See Policies / Etiquette for some ideas about what other reminders might be useful for your list.

There are some list settings you can adjust to help prevent large quotes from being sent to the list, should people forget.

You can adjust the maximum size of messages sent to the list at http://lists.uua.org/mailman/admin/listname/?VARHELP=general/max_message_size. You might try making your maximum message size 28 KB or so, if your digests are set to go out when they reach 30 KB.

You could also try filtering messages that include the word "digest" in the message's subject line (see http://lists.uua.org/mailman/admin/listname/privacy/spam).