Sue the debt collectors in small claims court. There are several example stories around the internet, but this is a well written one from the consumerist.
If your phone is a cell phone: "it
is against the law for a company to
leave a pre-recorded message on your
cell phone."
In fact, the call frequency increased
once they realized they had reached a
live person. I called each of these
companies multiple times, and though I
was given assurances each time that my
number would be taken off of their
lists, the calls continued, morning,
noon and night. At my wits end, I
decided the only way to have the
harassing calls stop was to file suits
against the collection companies. It's
very important to understand that it
is against the law for a company to
leave a pre-recorded message on your
cell phone. Armed with this knowledge,
I filed suit against several of the
collection companies. I filed in small
claims court so I did not need to hire
an attorney, and the process was as
simple as completing a paragraph on a
complaint form. For evidence, I had
over a hundred Google Voicemail
transcripts showing the times the
companies called and the text of the
pre-recorded messages.
Mysteriously, the calls all stopped
immediately on the same date the
collection companies received the
certified letters stating they were
being sued. Then a new flurry of calls
began pouring in. This time it was
their attorneys.
The attorneys representing these out
of state collection companies were all
desperate to settle out of court. hey
did not want to incur the expense of
traveling for court or hiring a local
law firm who wasn't on retainer. They
also understood they had no
justifiable defense for the calls. To
make a long story short, so far I have
successfully sued 3 of these
collection companies and settled for
more than $5,000 out of court. All it
cost me was $35 and 20 minutes per
suit.
Making these companies pay is the only
incentive for them to stop their
illegal and harassing practices. If
more consumers knew their rights and
actually took a few minutes to stand
up for them, it would become less
profitable for these companies to
conduct business the way they do now.
-Source
And whether you have a cell phone or land line, It is illegal for the debt collectors to tell you they are calling to collect a debt for someone else under the Fair Debt Collection Practices Act (wikipedia, ftc docs).
What Remedies Are Available If The
Debt Collector Violates The Law Under
the Fair Debt Collection Practices
Act, you have the right to sue a debt
collector in state or federal court
within one year from the date of the
violation. If you win, you may recover
damages in the amount of any losses
you suffered as a result of the
violation, plus an additional amount
of up to $1,000.00. You may also be
able to recover court costs and
attorney fees.
If the same debt collector has engaged
in unlawful conduct with a number of
consumers, it may be possible to find
a lawyer who will file a class action
lawsuit.
-Source
With regard to whether you can sue under FDCPA if you are not the debtor, one FDCPA lawyer (take with grain of salt) says yes:
Did you know that it doesn't matter if
you owe the account the debt collector
is calling you about or not? If a debt
collector violates the FDCPA (the
federal Fair Debt Collection Practices
Act, 15 USC 1692 et. seq.) that debt
collector could be liable to pay you
statutory damages, actual damages,
attorney's fees, and court costs.
-Source