Idk exactly if this is the way but this did the job for me...
- Press Windows + X keys from the keyboard, select Command Prompt (Admin).
- Type net user and hit Enter to list the user accounts on your device and I think the sandbox user starts with "itch-(followed by some numbers)" .
- Type net user "User account " /delete (replace user account with the "itch-") hit Enter to delete the user account .
- You'll find the folder of this "itch-" user in <drive>\Users\