This is a something that Apple has hidden away, leaving some Mac users out there to go through all sorts of lengths to remove items from their Time Machine backups.
The solution is really easy, though.
Open time Machine. Go to the top level -- the first window; the one with your hard drive. Navigate to the backup you want to delete and use the Actions menu (gear icon) in the Finder's toolbar. It has an option to delete the backup. Also, when using the Actions menu, with a file or folder selected, there is an option to delete all instances of the file/folder from all backups.
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