Dog in backyard

We are new homeowners. We are yet to fence our backyard because we haven't got our land fully yet and will be getting the extension once the land is levelled out. I'm sitting by the window and someone's pitbull is out there roaming freely. I generally love dogs but I don't really trust other's dogs. And I don't like dogs roaming freely coz they are unpredictable (not all I know, I brought one up myself that died a couple of years ago). What can I do in these cases? I had noticed this some time ago too, some dogs just roam freely in the neighborhood.