How can I completely remove all files from the device I recently installed a custom rom over my Android after flashing, removing all user data and wiping dalvik cahe etc.

But after installing it I ind out that the old files are still there such whatsapp folders and other apps also.

Why is that ? how can I completely remove  everything from my device and reinstall and fresh rom.
In addition to that, go to Settings > Storage  and touch  Erase xxx Storage for all the types of storage. That should wipe all your stored files.