Can't restore/reformat my USB flash drive after moving to GPT

I have 8GB flash memory stick. Sometimes ago I formatted it to be able to install Windows from it. I can't remember what exactly I did but I think I wanted to be able to install Win 7 64-bit to boot in EFI mode.

Now I can't reformat it nor in Windows nor in Linux.

I tried to convert it to MBR from GPT with gdisk by gdisk /dev/sdc, then w (to write GPT from memory to the drive as the tool reported "Found invalid GPT and valid MBR; converting MBR to GPT format in memory"). Then gdisk /dev/sdc again:

, opened expert tools with x, z (to destroy GPT). Here is the output:

then tried to delete a partition with fdisk

After this I tried to pull out and plug in the stick again but NTFS mount error window appeared each time.

i.sstatic.net/HocXv.png

I also tried to rewrite an initial drive area with a test content with badblocks

and with dd

but I couldn't reformat it to usable state.

Is there a way I can restore its factory state?

My system: Linux Mint 17.3

UPDATE