Syntax: WBADMIN DELETE BACKUP {-keepVersions: | -version: | -deleteOldest} -backupTarget: -machine: -quiet Description: Deletes ...

Syntax: WBADMIN DELETE BACKUP
  {-keepVersions: | -version: | -deleteOldest}
  [-backupTarget:]
  [-machine:]
  [-quiet]

Description:  Deletes the backups that you specify. If the 
specified volume contains backups other than backups of the local
server, those backups will not be deleted.
To use this command, you must be a member of the Backup Operators group 
or Administrators group. 

Parameters:
-keepVersions  Specifies the number of the latest backups to 
                keep. The value must be a positive integer. The option value 
                -keepVersions:0 deletes all the backups.

-version       Version identifier of the backup in MM/DD/YYYY-HH:MM format.
                If you do not know the version identifier, at a command 
                prompt, type:  WBADMIN GET VERSIONS. Versions that are 
                exclusively backups can be deleted using this 
                command. Use WBADMIN GET ITEMS to view the version type. 

-deleteOldest  Deletes the oldest backup.

-backupTarget  Specifies the storage location for the backup that you want 
                to delete. The storage location for backups is a 
                drive letter, a mount point, or a GUID-based volume path. This
                value only needs to be specified for locating backups that are 
                not of the local computer. Information about backups for the 
                local computer will be available in the backup catalog on the 
                local computer.

-machine       Specifies the computer whose backup you want to 
                delete. Useful when multiple computers were backed up to 
                the same location. Should be used when -backupTarget is 
                specified.

-quiet         Runs the command with no prompts to the user.

Remarks: One, and only one, of these parameters must be specified: 
-keepVersions, -version, or -deleteOldest.

Examples: 
WBADMIN DELETE BACKUP -version:03/31/2006-10:00
WBADMIN DELETE BACKUP -keepVersions:3 
WBADMIN DELETE BACKUP -backupTarget:f: -deleteOldest