Go ahead and have it plugged or patched and checked for leaks...then replace it with a new tire. Keep it as an unmounted spare in case you have to wait days or weeks for a replacement tire in the future. We bought the insurance on our tires at discount tire and have already replaced one back tire for $50 due to a nail. I never buy insurance or maintenance agreements on small stuff but these tires are too easily ruined and so expensive to replace. We've had three nails and two ruined tires in just 10,000 miles.