You can use the DeleteBackup method of the IIsComputer object to delete a metabase backup from a backup location.
IIsComputer.DeleteBackup BackupLocation, BackupVersion
MD_BACKUP_HIGHEST_VERSION | Delete the highest existing backup version in the specified backup location. |
<%
Dim ComputerObj
Set ComputerObj = GetObject("IIS://LocalHost")
'Delete version 1 backup.
ComputerObj.DeleteBackup "MyBackups", 1
%>