What is the process to sue debt collectors?

I have to disagree with the first poster on several points.
The FTC is not slow (well, they are but…). The FTC does not step in and do anything on an individual case by case basis. They generally take the complaints and act after there is numerous complaints against a company.
Even though the FTC does not intercede on an individual basis, it is still a good idea to have the FTC complaint filed.
You should also file complaints with the BBB, your AG and the collectors AG.

A collector does not have to cease calling unless the request is made in writing. After a collector receives a cease desist letter they are allowed one more phone call. That call cannot be for collection purposes but to tell the alleged debtor what their plans are for the alleged account - and in your case, if they call one more time it cannot be for the purpose of locating that person but to tell you they will no longer be calling.

The letter that was posted does not fit your situation in any way and I would not recommend sending it.
That letter tells them to cease collecting on the debt. They are not calling you in the attempt to collect the debt from you but rather to find the alleged debtor. Asking them to cease collections on a debt they are not even trying to collect from you will not get you very far with the court if you sue.

Your letter should be more along the lines of:
This letter is in regards to your telephone calls to locate [Jane/John Doe]. You have called numerous times in your attempt to locate [J/J Doe]. My responses to you included the facts that [J/J Doe] does not now, nor has ever, lived at this address, that I do not know of this persons whereabouts and requests for you to cease calling me concerning this person.
I am requesting, in writing, that you cease further communications with me concerning [J/J Doe] as I find your telephone calls harrassing and inconvenient to me.

Sincerely
[you] (type your name or print your initials, never sign your signature to a letter that goes to a collector)

Send it certified mail return receipt

(the letter I posted is just a sample, if you use it then make it your own)

If they continue to call, file your complaints with the BBB and the AG’s. If they continue to call, learn your states rules of civil procedures for small claims courts and file a suit against them. You should also read the FDCPA.
Never embellish or lie - on the phone, in your letters to the collector, in your filed complaints. If you do end up suing and it is found that you have embellished or lied then not only will the judge toss your case and you would be liable for all court costs, but you may end up finding yourself in contempt of court (which, depending on your state, could mean a stiff fine and/or jail time)

You have to build a solid paper trail that proves to the judge that you have done everything in your power to resolve this before you finally decided to file suit.

As for the air horn comment by the first poster, while it would be great to blast them with it, you could very well find yourself sued by the collector for physical or emotional damages (yes it has happened) Your best bet, and what would look best to a judge, is if you handled the calls in a civil manner if they continue to call before and/or after you send the CD. (if you file against them and you had made verbal threats or did something that could possibly be harmful to a person - an air horn blast in the ear, etc, the collector more than likely records the calls and they may not hesitate to use it against you)

If you have caller ID, take photos of it every time they call. You might also purchase a recorder and tape their calls. If you tape the calls you need to learn your states taping laws - or - if you do not know your states taping laws you should inform the collector that you will be taping the call.