Old debt trying to be collected

I received a call today (on my cell) from Portfolio Recovery, something or other bottom feeders. It was regarding an 8 yr old debt that is long past the SOL here in Tx. I never admitted the debt, and told the woman I knew it was past the SOL, not to mention I knew they'd just bought the debt for pennies on the dollar, and that it can't legally be on my credit report since it's been this long. She also argued with me about how long the SOL was for TX, saying that every state was 7 yrs, and I told her she was wrong. She just said, "Yes, but the SOL applies to not being able to sue you, we can still call and demand the you pay the debt forever." In the heat of all of it, I said I wanted verification that this was my debt and that I wanted to see proof that I'd promised to pay them this debt. She wanted my address so she could send me this info, but I was afraid they'd take that as some sort of agreement that would start the SOL over, so I refused to give it to her just yet. (told her I'd call back) All that being said, what should be my next step? Based on what I've read so far, I'm thinking a C & D letter, but just want to make sure. Once I send that, they HAVE to stop all contact, correct? How do I go about making sure it's not reported on mine and my husband's credit reports? Is there some sort of script I should follow if they (or other bottom feeders) call again?