If a collection agency has contacted you about collecting some manner of debt and you know yourself to be clear of that debt, you are not doomed to the harassment that these collection agencies might force on you. If you get a letter in the mail from one of these agencies, you can do several things to dispute that company and cut off their legal right to contact you concerning that debt.
Contact the company that you are alleged to have debt from. If you have already paid off whatever debt you had, you can legally implore them to stop the collection agency that they hired from contacting you. This method does not always work because sometimes credit card companies take time to process payments and will not cancel collectors until all the payments have gone through, which can take months.
Send a letter to the collection company, formally disputing the charges. If it is the first time you have heard about the debt, which is a common occurrence in such situations, state that it is the first time that you have received such a notice and that, in accordance with Fair Debt Collection Practices Act, Section 809(b), this voids you of any prior obligation to pay the debt by the specified date on the letter that you did receive. Sending a letter of this nature in will usually stop the collection agency from pursuing the matter further.
Make sure to enclose your current forwarding address on the letter to make sure that the collection agency can contact you if it has any issues. If the collection agency tries to contact you, but cannot reach you, this will make it more suspicious and more likely to pursue the matter fully. Though it might be an annoyance, being truthful with a collection agency goes a long way.