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

 logical-name[:]

    Specifies the logical name to be deassigned. Logical names can
    have from 1 to 255 characters. If the logical name contains
    any characters other than alphanumerics, dollar signs ($),
    or underscores (_),  enclose it in quotation marks (" "). The
    logical-name parameter is required unless you use the /ALL
    qualifier.

    If the logical-name parameter ends with a colon (:),  the command
    interpreter ignores the colon. (Note that the ASSIGN and ALLOCATE
    commands remove a trailing colon, if present, from a logical name
    before placing the name in a logical name table.) If the logical
    name contains one or more trailing colons, you must append one
    additional colon to the DEASSIGN logical-name parameter (for
    example, type DEASSIGN FILE:: to deassign the logical name
    FILE:).

    To delete a logical name table, specify the table name as the
    logical-name parameter. You must also use the /TABLE qualifier to
    indicate the logical name directory table where the table name is
    entered.