Contact a local attorney who files cases under the Fair Debt Collections Practices Act.
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If the Bill Collector is not the creditor you allegedly owe, but is an outside bill collector, then you probably have a good claim under the FDCPA, so long as the bill collector received notice in writing to cease contact with you.
I would definitely save a copy of the letter you sent (hopefully you already did this) or send another notice, via certified mail, to them (keeping a copy of it) that you are disputing the debt and they should no longer contact you about it. This does not eliminate the debt, but it would prevent them from calling you and harassing you about the debt.
These cases are typically done in federal court, and often result in relatively quick settlements without a trial, IF the consumer has a strong case.