i'm sorry to hear this is happening to you. it sounds like the creditor is violating the FDCPA US Title 15 Section 1692. this consumer law protects us against creditors abuse which it appears this company is doing.
first of all, record the calls that they make to you outside of business hours - especially the late night ones. you can use this as evidence.
there are several letters online that you can send to the creditor requesting that they cease and desist this act. if they do not, they can be fined up to $1,000 I believe.
hope that helps!