how to stop iphoto from organising photos (physically relocating)?
Go to iPhoto → Preferences → Advanced  and uncheck Copy items to the iPhoto Library.

Note that iPhoto will still copy files to its library if you import photos from an SD card, etc. Instead, copy them where you want first then import them.

Also, changing this setting on an existing library will not relocate media that has already been imported. You can create a new library by holding ⌥ whilst opening iPhoto.