I think aggression is justified. With this sort of thing you really don't know how far people will go, when it will end... So if you don't write a really scary note, people might not be convinced that you really mean it and will consider to try and get away with violations that they consider to be negligible.
My staircase-mates have been using my teaspoons and not washing them up. I'm not too fussed because it doesn't really build up to anything that much of a pain to clear, and I haven't lost any of the spoons. But if my cutlery starts to live in their room, then there will be words!