CC company still making threats even though I'm paying them something

F'n Citi! These low down scum bags will stop at nothing. I am currently paying them via a payment arrangement that THEY set up and authorized. They call non stop, just like they did prior to setting up the arrangement. They have called my co workers at my job and left threatening messages on their voice mails telling them, "tell this dead beat that we will aggresively pursue him by whatever means are necessary!" Whenever I speak with them myself, I ALWAYS inquire about the arrangement that I am currently in, and they get pissy and say "that's not good enough to satisfy this debt and you know it, so stop stalling and pay up, or you will be in cuffs sooner than later!". I have asked them to stop calling work. They persist. They have told me that they have spoken with the actual sheriff in the jurisdiction I reside in and he told them to tell me to consider these calls an official "law enforcement warning". Meaning that they will get an warrant and arrest me on the spot if need be. They have even implied injury or death in a thinly veiled way. saying things like "accidents do happen". Is ther just NO regulating these a holes? Are they in desparate times due to the economy and so they're using any tactics they want? And what about the payment arrangement I set up with them? Can they still screw with me even though I'm paying them something every month?