Below is how to securely delete files that cannot be deleted you can try. You can use third-party programs to securely and permanently delete a file such as following.
How to securely delete files that cannot be deleted with using software
Eraser. This tool can completely remove sensitive data from your hard drive.
Freeraser. This tool is a free file shredder that will render sensitive information unrecoverable.
DP Shredder. This tool can delete or destroy files and folders from your system.
File Shredder. This tool can remove files from your hard drive without fear they could be recovered.