("I paid the Post Office five dollars extra when I mailed this so that I have admissible proof you got the letter when I sue you for ignoring its contents.")
I used the (well-founded) threat of it to resolve some difficulties after some creditors got the mistaken impression that I owed them money. One letter to get them to stop calling me, one more letter to remind them that the three calls after the first letter mean they owe me $3,000 under the Fair Debt Collection Practices Act and the meter is still running if they want to call me again.
My only regret is that I didn't have enough days off that year to march down the court and actually sue them. I think its... salutary to remind debt collection agencies that the legal system works both ways. For one, if more people did it I'd get less people thinking I owe them money because I share the same first initial, last name, and (former) zipcode as someone who might.