How do you stop people talking gifts left by others

You can't really. You just have to hope you get polite players come to you island.

Someone gifted me money, and two others took the whole lot

Oh, that is low. I would never do that. Can other players drop items in your house? Because then nobody would be able to pick it up then. Somebody almost took some stars I was giving to a host once, but I stood over it.