/sys$common/syshlp/helplib.hlb SET, TERMINAL, Qualifiers, /BROADCAST  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /BROADCAST (default)
       /NOBROADCAST

    Controls whether reception of broadcast messages (such as those
    issued by MAIL and REPLY) is enabled. Specify the /NOBROADCAST
    qualifier when you are using a terminal as a noninteractive
    device or when you do not want special output to be interrupted
    by messages. Use the SET BROADCAST command to exclude certain
    types of messages from being broadcast, rather than eliminating
    all messages.