Ok so they are debt collectors and as debt collectors then they are subject to the Fair Debt Collection Practices Act which really prohibits and does not tolerate this type of behaviour.  They are not only harassing you but they are threatening you. They are not permitted to do so.  Threatening arrest is also a big no no.  I would call and report them to your state attorney general's office and to the Federal Trade Commission as soon as you can.  You absolutely do not have to put up with this type of behaviour and you should not for one more minute.  However, you will have to eventually deal with the bad check.  That may be a really good idea as soon as possible. Good luck.