You don't say where you live and rules change country to country and sometimes state to state (if you're in the US)
I'm in the UK, but I know the US has a similar system, just get some legal advice on how to apply it.
First of all, the credit company has to be able to prove they own the debt and that you are the debtor. Often when old debts are bought the paper trail goes a bit astray.
Do not acknowledge the debt or make any payments before asking the credit company to prove ownership of the debt, to prove that you made the original agreement to pay the debt (i.e provide a copy of your agreement with signature) and provide a detailed account of charges and arrears.
Do all of this in writing by recorded or certified mail that requires a signature on delivery. Keep your letters and mailing receipts. When you write to them be very clear that you dispute the debt and will not pay it at the current level.
Be very clear that you do not want phone calls or personal visits and that all communication with you must be in writing. Once you have told them that, they have to stop harrassing calls and visits.
If you have receipts of payment of the original debt send them COPIES not original documents.
Because of the age of the debt it may be too old to collect anyway, seek some legal advice for your country.