I have scam-artists calling me with spurious credit claims, saying I owe them money. How can I make them stop?

I messed up my credit as a kid, and it's taken me years to rebuild, but now I'm getting phone calls from people who barely speak English and have only a 1-800 # and a PO Box, wanting to "collect on a debt" that's over 6-7 years old and this is the first time anyone was able to get a hold of me ~ obviously, a scam...but what are my options for making them go away? I've called the BBB on one company - turns out they've already had 15+ complaints brought against them, and their rating is so low they don't even care about the Better Business Bureau. They call daily, nighly, on weekends & holidays, usually leaving recorded messages...Do I have any protection under law without pursuing litigation or changing my phone number?