I have encountered another Bug in SmithCart from the version up to 5.00 to 5.24. This is also in your test database.
Problem: I have unchecked a coupon to be Active in my store, but, while the box is unchecked, status should be disabled, I am and other customers are still able to use the coupon. The only method I have found to stop this was to set the Coupon to an previous date to mark it as expired and disallowing users to use that coupon even though it was marked for Disabled and Inactive.
Pictures are below and are from your test database.
I do not check the Active Box
Yet the coupon is still applied to my order.
You can try this and the same couponcode in your test site area.