You can stop Livedrive from backing up individual items from your backup by deselecting the items from the Manage Backups window.
Removal of backup sets can be actioned through either the Web Portal or from one of your mobile devices that has the Livedrive app installed on.
Login to your web portal
By default you are on the Devices page
Select the backup set you wish to remove and click on the name
Enter DELETEME and press OK
To remove the backup set from the web portal, click here
To remove the backup set using a mobile device, click here