As soon as they call, I connect AND then I hit every number on my keypad continuosly until they hang up. I've been told if it's a computer calling, it will recognize your response as a non working number. If it's a live operator, they have to believe it's not a working number, too. Either way, I'm told your number will be kicked out of the system.
Did it to a pair of Florida numbers in the last three days and no call backs.