Make Time Machine backups erasable
Simply use Finder to drag a copy of the folders you want to maintain out of the Backup.backupdb hierarchy and then delete the copies left in Time Machine or delete all backups.


How can I manually delete old backups to free space for Time Machine?
http://support.apple.com/kb/HT1427 - You can also enter the Time Machine restore interface and find files that can be removed from the backup drive itself to conserve space. To do this, select the file(s) and from the Action pop-up menu (gear icon) in the Time Machine Finder window choose "Delete All Backups of...". Be sure to only delete files you are sure you won't need or want to restore later.