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

 device-name[:]

    Name of the device to be deallocated. The device name can be a
    physical device name or a logical name that is not in use. On a
    physical device name, the controller defaults to A and the unit
    to 0. This parameter is incompatible with the /ALL qualifier.