Note that I am not licensed to practice in your state.
You need to make sure that you truly waited out the SOL and that during that time, you didn't do anything to reset the clock. Things that would reset the clock are acts such as giving them a check or making a payment, promising to make a payment.
You need to also make sure that they did not get a judgment against you before the SOL period was up. If they did, then they have the right to come after you now and anytime in the future.
If all those things are met and you truly waited out the SOL, then you need to write them a Fair Debt Collection Practices Act violation letter stating that they have passed the SOL to collect this debt and that you are no longer obligated to pay it. They should stop contact with you or else they will be in violation of federal law.