Robocall left message that "? sixteen hours and once it get expired after that you will be taken under custody by the local cops as there are 4 serious allegation pressed on your name at this moment. We would request you to get back to us so that we can discuss about this case before taking any legal action against you the number to reach us is 646 809 9640. I repeat 646 809 9640. Thank you."

Got the exact same message as above from this number.

Which agency can trace the calls and find out who is scamming us?
There must be a way to shut them down or at least make it more difficult for them to operate.

Just got the same call, this is illegal!

There's no way to prevent the calls from being initiated, or pinpoint the location of the callers give current technology.  Callers are located in India and Pakistan.  Your best bet it to invest in a call blocker and NOT pick up on call numbers you do not recognize.  You might want to block the entire 646 area code; it is often used by these kinds of scammers.