Thanks for the prompt reply Emiel1!
In fact, my listing is also currently marked as unlisted...so it is a bit unclear how these people got to it.
Concerning the "instant book" option, I went to look, and it is indeed on...possilby my fault then (however this listing has been unlisted for a lot of time now, and I don't remember...but it feels kind of strange that I would set up an instant booking for it...).. thanks again.