Use the DELETE/HISTORY command to purge history lists.  You have  the
  option  of  specifying  the number of entries or the creation date of
  the earliest entry you wish to save.

  Command Syntax:

    DELETE/HISTORY [qualifiers] path-name [,path-name]...

1  –  privileges

  You need PASS_THRU, LOCAL_DELETE, and SEE at  the  target  dictionary
  directories, subdictionaries, or objects.

2  –  qualifiers

2.1    /BEFORE

  Syntax:

      /BEFORE = time-specification

  Use /BEFORE to specify the creation time of the oldest  history  list
  entry  you  want  to  save.   All  entries  made before that time are
  deleted.

  Time is specified as the  VAX  standard  time  of  day:   dd-mmm-yyyy
  hh:mm:ss.ss.   You  may  omit any of the fields, but you must include
  the punctuation (hyphen, colon, period) following any  leading  field
  you omit.  Omitted fields default to the current time of day.

  You cannot specify both /BEFORE and /KEEP in the same command line.

2.2    /KEEP

  Syntax:

      /KEEP = number

  Use /KEEP to specify how many history list entries you want to  save.
  DMU  then deletes all but that number of the most recent history list
  entries.

  You cannot specify both /BEFORE and /KEEP in the same  command  line.
  If you do not specify either /BEFORE or /KEEP, the default is /KEEP =
  0, and all history list entries are deleted.

2.3    /TYPE

  Syntax:

      /TYPE = (type-specification [, type-specification]...)

  Use /TYPE to specify a particular type  of  dictionary  directory  or
  object whose history lists you want to purge.

  A type specification names a type, such as DTR$DOMAIN or  CDD$RECORD,
  whose  history  list  entries  are  to  be  deleted.  You can use the
  wildcard characters % and * in a type specification.  The parentheses
  are optional if you specify only one type.