Will's correct. There's no proper way to remove files from images.
Assuming your backups are from clients that still exist, the only way I can think of to remove a single file would be to create an exact duplicate of the original client including the original client's name (if you used BMR to back everything up, this would be ideal) in a separate, alternate network, restore the entire client from the existing backup to the duplicate client, delete the individual file from the client, then back up the duplicate client. You could then expire the original images.
Note, however, that the expiration date for the new backup images would need to be adjusted to expire at the correct time. And the dates on the backup would now be the current date, and not match the original backup. Which would basically be madness trying to figure out what happened to an entire week/month/however long of backups to restore from in the future.
This is the same reason companies like Google, Microsoft, etc. have to tell you they cannot completely purge your data from their systems, because your data will always reside on a backup somewhere and there is no way to remove a single file from a backup image.
