I would say something, even if it was just a note on the windscreen.
Say something if it bothers you that much - even a casual, confused, non-threatening kind of wondering why they prefer out the front of your place to theirs lately etc but I don't think a windscreen note will go down very well. It's too easy to misinterpret. A friend of mine who came over a few months back to help out for the day parked partly outside my house and partly outside the next door neighbours place. I was out front of my place and coming and going all day. Anyway she got a note from the neighbours on her windshield asking her not to park in front of their house. It was not well received by me (not that I did anything about it, they were renting and have since moved and I was in the middle of a crisis and didn't want to over-react) but she had every right to park on a public street wherever there was space.

So to answer your questions, the council/RTA actually own the street, curb, nature strip, footpath etc (whatever applies) and you have no legal right to stop them parking in front of your house. Why not park in front of their house if the spot is always vacant?