UGH I have the same problem!!!!! Unfortunately, my roommate feeds her cat Dry Meow Mix (eww) and her fatty cat eats all of my kittens nutro (speak of the devil, he just came into my room, looking for food again)!!! I can't convert my roomie to something better either, because she can't afford it (or just doesn't care). I don't know if there's much you can do about it. I usually just keep my kittens food bowl in my room and put it away if I see my roomies cat come in my room.
Iams isn't horrible, but there are better brands. I had to buy a bag of Iams a couple weeks ago. I started putting out the Iams during the day, knowing my roomies cat was going to eat most of it anyway. I put out Nutro for my kitten, at night, when both he and the bowl are shut in my room. Luckily, my kittens tummy adapts easily to change, so I'm able to rotate his food like that.