Eventually I did figure out that on the website you can say you aren't interested in particular items, but Android/Roku etc do not have that option. Of course those Netflix employees who work on this stuff deal with the PC/web based interface all the time, but apparently don't realise that not everyone does that.
Android/Roku etc do let you set ratings on items, but to me there is a big difference between "I am not interested in this item" and "I have watched it and give it one star".