OS X Yosemite: Remove a backed-up item from your backup disk
You can delete all copies of a backed-up file or folder from your Time Machine backup disk.
-
Click the Time Machine icon
in the menu bar, then choose Enter Time Machine. If the Time Machine menu isn’t in the menu bar, choose Apple menu > System Preferences, click Time Machine, then select “Show Time Machine in menu bar.”
-
Use the arrows and the timeline on the right side of your screen to browse the backups Time Machine created.
-
Select the item you want to delete, then click the Action menu
and choose Delete All Backups of [item name]. 
You can move, copy, or delete the Backups.backupdb folder on your Time Machine backup disk as you would other folders. You can also delete folders within the Backups.backupdb folder. You can’t delete individual items inside the dated folders.