How old is the SD card?
How old is the SD device?
When you reformatted, did you run a blocks check on the card to make sure it's still good?
What device are you using to delete the files?
Have you tried using another device to delete the files?
What type of filesystem is on the SD card?
What are the exact locations and names of the files that can't be deleted?
What is the exact error you get when trying to delete the file?