How can I prevent my drug addicted nephew from coming to my aging parents home?

You must notify the authorities and probably obtain a restraining order as a start. You must also change locks and do all you can to insure he physically can’t “move himself in”. Do you know their neighbors and can they help watch out? So sorry for you, this is difficult.

Yes, restraining order at the very least. A person with a drug habit will want to stay as far away as possible from the law. You also need to make sure your parents understand that they are not to even talk to him. They might feel badly but if you tell them that the *police* say he can't be around they won't have to feel guilty about it. Have you talked to the local police, just to let them know about this situation? They may not be able to do anything but might be able to refer the situation to an adult protective services organization that can give you some direction.

If you have the technical ability, wireless cameras are fairly cheap these days. You can install the app on your phone and monitor your parents home at any time so if you see him around you'll know to act immediately.