It's against federal law for a bill collector who works for a collection agency (as opposed to working in the collections department of the creditor itself) to call you at an unreasonable time. Before 8 a.m. or after 9 p.m. are considered unreasonable times, but other hours may be unreasonable, too, such as daytime hours for a person who works nights.
The federal Fair Debt Collection Practices Act (FDCPA, 15 U.S.C. § 1692 and following) bars collectors from:
Under the FDCPA, you can demand that the collection agency stop contacting you (except to tell you that collection efforts have ended or that the creditor or collection agency will sue you). Make your request in writing.
For more information on what the FDCPA prohibits and what to do if a collector violates the law, see Nolo's article What to Do If a Bill Collector Crosses the Line.