Barclays have recently sold a lot of debts to Lowell/Red. If your contact is from them, they know that a lot of these debts are statute barred.
Ethel is right, although these firm's idea of what does or does not constitute harassment is very different from most people's!
You can tell them it is statute barred & that you can't/won't be paying it & see what happens. If they do go on hassling you you could agree a very small payment in full and final settlement (they will have paid peanuts for the debt so should settle for a fraction of the amount).