VMS Help SET, DIRECTORY, Qualifiers, /EXCLUDE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /EXCLUDE=(filespec[,...])

    Excludes the specified directories from the SET DIRECTORY
    operation. You can include a directory but not a device in the
    file specification. The asterisk (*)  and the percent sign (%)
    wildcard characters are allowed in the file specification;
    however, you cannot use relative version numbers to exclude a
    specific version. If you specify only one directory, you can omit
    the parentheses.