VMS Help TCPIP Services, Programming Interfaces, Sockets API, $QIO, IO$_TTY_PORT|IO$M_TN_SHUTDOWN, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The IO$M_TN_SHUTDOWN subfunction allows the application to break
    a previously bound socket-terminal connection (created with IO$M_
    TN_STARTUP). The channel must be from an assignment to the same
    network pseudodevice in the socket-terminal connection.
    Upon completion, the application retains the assignments to
    the connection and the TELNET terminal, but they are no longer
    related. Any subsequent IO$_READxBLK or IO$_WRITExBLK function on
    the socket channel will no longer return the error SS$_DEVINUSE.