Hi @laczen,
on Windows it will require you to remove the folder, C:\user\YOUR_USER\.platformio. On linux it will be /home/YOUR_USER/.platformio.
If you installed PlatformIO IDE into VSCode (or Atom), then just delete the extension in the usual manner. On VSCode this will be clikcing the uninstall button when looking at installed extensions on the left.
HTH
Cheers,
Norm.