Maybe you can send them to Washington, DC ?
Or you could keep them local and send them to the District Attorney's house since they were supposed to stop these kinds of calls from getting through?
Lastly, I'd go with a whistle or an air horn!
If you were to go the route of mailing them a "care-package", there was an example of a good one from a book called "Dear Sir, Drop Dead: a collection of hate mail through the ages!"
The sender sent a box of chocolates (small) inside a larger box stuffed with crumpled newspapers and cockroaches. They will discover the "added" gift too late to prevent them from scurrying all over the place.
Just make sure to give a return address that is a vacant lot (for your security)!