We are seeing customers purchase the Intro Class special multiple times. Can we block it so customers can only purchase this package once?

The 'Customer Limit' field on the Purchase Options section of the Series form is what you're looking for. Setting it to 1 will limit to one purchase per customer. And, if you suspect that customers are using different email addresses (creating multiple accounts) to take advantage of the special, user Reporting > Duplicates to try to track this down.