Discount codes are created when someone opens an email for the first time.
It is possible that customers are opening emails that were previously sent before the discounts were removed.
For example, If the email was sent 1st and the customer opens the email on the 13th the discount code will be generated from the 13th. This code will run for the allocated time of your discount codes that was configured in the Spently App. If this was for 7 days, the customer will be able to use this code until the 20th. You can manually disable these codes in the Shopify backend.
However, these discount codes are 1 per person, so they can't be stacked with other discount codes.