You haven't said how you know they are doing this but, firstly, if you can prove this is happening, then report them. It also depends on how far away from them you are. If your properties share adjoining walls, then it is possible. If they are across the street, it is unlikely they can listen to you. Install a security camera on the outside of your property, pointing in their direction. Look into setting up some kind of white noise blocker at the nearest wall or window - you can do this easily with a radio or a device playing an appropriate YouTube video (there are lots of them that play for hours at a time). It doesn't have to be really loud but it will ensure that your conversations are indistinct. Make sure any important private discussions take place in your home as far away from the neighbor's property as possible.