Another gem I found somewhere.
Permanently Delete files from Time Machine Backups
While navigating Time Machine, go to the present view, and click the little gear icon in the toolbar of the Finder while you have the file you want to delete selected. Simply click the option to Delete all Time Machine backups of this file, and the file will be gone forever.
Then, to make Time Machine not back up that folder/file again, go to Time Machine preferences, click on Options then the + to select the files/folders/drives you want to exclude.