Among other annoying things, the jerks kept asking for my SSN as a way to confirm that I wasn't the person they sought. I explained that like most people with good credit I don't give out my SSN to any random person who calls me.
Unfortunately you have less protection as an uninvolved party than you would if you owed the debt because of the way the Fair Debt Collection Practices act is written, but I always mention the act and tell them that I am invoking my rights under the act and their repeated calls after they have been told it's not the right number constitutes harassment.
(the section of the law is § 806. Harassment or abuse <15 USC 1692d>)
A debt collector may not engage in any conduct the natural consequence of which is to harass, oppress, or abuse any person in connection with the collection of a debt. Without limiting the general application of the foregoing, the following conduct is a violation of this section:
(1) The use or threat of use of violence or other criminal means to harm the physical person, reputation, or property of any person.
(2) The use of obscene or profane language or language the natural consequence of which is to abuse the hearer or reader.
(3) The publication of a list of consumers who allegedly refuse to pay debts, except to a consumer reporting agency or to persons meeting the requirements of section 603(f) or 604(3)1 of this Act.
(4) The advertisement for sale of any debt to coerce payment of the debt.
(5) Causing a telephone to ring or engaging any person in telephone conversation repeatedly or continuously with intent to annoy, abuse, or harass any person at the called number.
(6) Except as provided in section 804, the placement of telephone calls without meaningful disclosure of the caller's identity.
Whether they are required to desist because of this I don't know because I'm not a lawyer, but most of them back off when they hear the magic words "Fair Debt Collection Practices Act."
I know that they have a tough job tracking down people who skip on debts but that's no excuse for abusing the rest of us. By the way, if "Mary's" number is listed in your phone book the debt collectors are also morans because Telephone Tracing 101 begins with calling the listed numbers. I used to trace people for a living (not as a debt collector!)