There appears to be a limitation in the handling of Out of Office notifications (OOF) by the Microsoft Exchange Distribution Lists (DL) - if the message is received by the DL as a "raw" SMTP message, the fact that "Allow out of office messages to originator" is selected in the DL properties seems to be ignored.
In order for the OOF messages from DL members to be suppressed by de-selecting the "Allow out of office messages to originator" option, the originating email has to be sent as an "internal" Exchange (IMAP?) message. Therefore, if you have any Perl/ASP/CDO application sending SMTP messages or if the messages go to/from recipients that are still on Exchange 5.5 (these are converted to SMTP traffic when forwarded to Exchange 2003/2007) - OOF notifications from DL members will still be fired off to the "originating" email address.
Unfortunately, the only thing you can do is make sure that you de-select "Allow out of office messages to originator" in the Distribution List properties and hope that nobody sets your address as the "Sender" for their SharePoint application.
P.S. This is also the reason why people that receive spam (or are members of Distribution Lists that receive it) while they're out of office, tend to get placed on many other spam lists - spammers receive their Out Of Office notifications, which confirm that their address is a valid recipient. If you want to avoid this - use custom Rules instead of Out Of Office assistant.