MMarble,
Make sure that the flag Designate This Mailbox for Play-Only is set to Yes.
Voice Processor > Devices > Mailboxes > 900: Designate This Mailbox for Play-Only: Yes
That way they won't be able to leave messages if they get there.
Also you may want to hide it from people using that are using the Directory.
Voice Processor > Devices > Mailboxes > 900 > Directory Information: Unlisted: Yes
Thanks,
TE