I tested this here and found that if you set the content rating to G on the device then this allows all content to be streamed to it because the minimum age for G is 'None' or 0. The setting for the Device is 'Do not stream files with the following content rating or higher' so setting it to G should prevent all files from playing as the content rating will test to see if the minimum age is less than 0, try setting it to 'M' or 'PG-13' if you are using US Content Ratings and this will prevent an R rated file being streamed to the device.