http://www.consumer.ftc.gov/articles/0149-debt-collection (Threatening civil proceedings with no intent to follow through, or threatening criminal process in order to collect a civil debt is a violation of the Federal Fair Debt Collection Practices Act.) The same goes for endless telephone harassment.
Note that the US Supreme Court in the Case of HENSON ET AL. v. SANTANDER CONSUMER USA INC., has recently ruled that the provisions of the Federal Fair Debt Collection Practices Act do not apply if the creditor owns the debt in question. As a practical matter this probably won't effect the kinds of scam debt collectors who often show up on 800notes. The scam collectors don't give you any information about themselves so it is impossible to know whether or not they in fact "own" the debt in question. Most do not send debt validation letters and many often say they are collecting for someone else. So, make the complaint anyway if they are bothering you. Let the FTC sort it out.
Also contact your local attorney general
If they continue to bother you, and you know you do not owe any money, tell them that you know this is a debt scam and that you are making a complaint to the police for extortion. Once you get a copy of the police report and they call you again , just read out the file number and the name of the PD or Sheriff’s office involved. Tell them you’ll be glad to send a copy to them at whatever address the jack***es are working out of now.
You'll probably have to yell at them and talk over them. Tell 'em no scammy debt collector is going to push you around.
You have to get tuff with these people because, being akin to thugs, brute force is (metaphorically speaking). the only thing they understand.