Is there a way to stop them?

Since there are no standards for out-of-office (OOO) messages, how they are handled depends entirely on how the remote mail server decides to handle the OOO reply. LISTSERV can recognize some (but NOT all!) OOO messages and deal with them (i.e., ignore them). Other OOO messages get sent to the error-handling address for the list, and this can potentially cause LISTSERV to eventually auto-delete the user whose mail server is throwing back the autoresponse. Other OOO messages get sent back to the list address and can cause loops, which are easy enough to fix--you just delete the user or set him to NOMAIL. Still other OOO messages get sent back to the person who made the original posting (since his address is in the From: line), and LISTSERV never sees them at all. These last are the confusing ones because they sometimes make users think that their messages aren't being accepted by the list, when in fact their messages are being accepted but are being bounced by some other subscriber's OOO setup after LISTSERV sends them out.

Many list owners don't even mess with people who set OOO messages and go on vacation. They delete or NOMAIL the user as soon as they start seeing the messages or start getting complaints from other subscribers.  How to handle this, of course, is a decision that should be made locally.