ISO settings doesn't control the shake or motion blurr in the image. ISO is the light sensitivity of the sensor. ISO400 seems to be a good number to start with though looking at the size of Spark sensor. Anything above that may produce too much noise in the image. Now coming to how to reduce shake, it depends on the shutter speed you select. A general rule of thumb in photography is that shutter speed should be at least equal to the reciprocal of the focal length. That means since the Spark camera has a 25mm focal length, your shutter speed should be 1/25sec or faster. Anything less than that may produce shake in your image. So I would suggest you start with ISO 400 and shutter speed 1/25. Remember that the output would totally depends on the lighting of the place where you wish to photograph and depending upon the conditions, you may have to adjust the shutter speed or ISO.
Please note that I have no experience in night photography with the Spark since night time drone flights are illegal in Japan. All I shared above was my general knowledge about photography.
