How can I delete Windows7 Backup

I have been using the Windows7 backup on an Acer x3910. I now want to pass this to another person, but want to delete the backup details of my accounts etc. I have tried –

a) Renaming Windowsimagebackup and xxxx-pc onj the backup disc (A logical disc on same physical drive as the live disc), so that new folders are created when I next backup.. but after the new backup the folders that have been removed on the live disc are still on the backup.
b) I have turned off system protection … but the folders are still there when I go to backup.

Surely this will be a problem shared by others, but there does not appear to be an obvious solution .. any help appreciated..