I'm in favor of having Settleup and other folks with similar viewpoints spouting off. It lets you know where they're coming from, and I find that valuable information even if the factual or legal statements are not. Settleup does make a good point, although in a somewhat abrasive tone, that there is no time limit within which a debt collector must respond to the demand for verification.
In my experience, most debt collectors simply are not set up to deal with demand for verification letters. So they pass the debt around to one after another, and the purported debtor has to send each of them a new demand for verification. I think they play that shell game in order to confuse the consumer on the assumption that sooner or later, one of them will send the consumer a demand for payment that will go ignored because he'll think he's already sent the demand for verification letter to them.
The lesson is to keep track of everything that happens with debt collectors in writing. That way you'll have a log of what you did when so you can keep yourself straight, as well as the ability to "refresh your recollection" if you need to when you file suit against the debt collector.
Each and every violation of the Fair Debt Collection Practices Act is good for up to $1,000 in statutory damages as well as reimbursement of costs, legal fees, and interest on the total at the judgment rate. If you tell them in writing not to contact you anymore, each subsequent contact is a violation. If you demand verification of the debt and they continue to try to collect without providing the information, each and every attempt to collect is a new and different violation. Keep track. They may easily end up owing you more than they say you owe them.
As to the sample form letter, I just posted one similar to that which I regularly use in the "Bureau of Collection Recovery" discussion area (I'm generally writing as an attorney for the purported debtor, so the language has been modified to suit a person who's writing on his own behalf). I don't claim any copyright, use it as you see fit.