This link suggests what mjd07 did and has follow up instructions should the collections agency continue on after receiving the letter.
Is there any way to stop collection calls for someone else? - Yahoo! Answers
There is no need to go through all kinds of measures like phone recordings, changing numbers or the like.
Simply send them the following notorized letter certified mail/return receipt (you fill in the information)
DATE: ___________________
TO: ________________________________________… ________________________________________… ________________________________________… (Name & address of debt collector)
FROM:
(Your name & address)
SUBJECT: Cease contact on debt regarding ___________________________ (Name of person the debt collection agency is contacting you about)
To whom it may concern:
Under the Fair Debt Collections Practices Act, I am exercising my right to request that you cease all contact with me about debt owed to you by [name of person the collection agency is calling you about].
That person does not live at this address and cannot be reached at this phone number. If your company can prove that this debt is mine or that this person is in some way connected to this address and phone number, please send proof of that claim to me in writing, again in accordance with the provisions under the Fair Debt Collections Practices Act.
Please also confirm to me in writing that you acknowledge this request and that your company will honor this cease action notification. If not, I will be forced to seek legal action and file a complaint with the Federal Trade Commission Bureau of Consumer Protection. It is also my understanding that your company has two weeks to respond to this letter.
Sincerely, ________________________
(Print name) ________________________
(Sign name)
You must sign to make this document legal and binding!
Here is the link to the FTC ito file a complaint in case they do not honor the letter
https://www.ftccomplaintassistant.gov/
Hope this answer is of help to you
LEGAL DISCLAIMER: The answer provided here is intended for informational purposes only. It is not intended nor presumed to be legal counsel or professional legal advice