NCO Collections review: What to do?

Over 15 years ago, Sears posted a credit complaint against my credit saying I owed them money. I research and they had some guy who has a similar last name - but different mailing address, different account number tied to my social security number. Contacted Sears and was told it was taken care of. 15 years later, being harassed by NCO for the same charge. Want to sue Sears! And NCO! What to do? I was told that this was handled over 15 years ago and now NCO calls me over and over again, even though I told them it's not mine