If you were using Zoom before you integrated it with TeamUp, it's possible some of your customers might have your old public link, which will still allow them to join classes.
If you don't want customers to use the old link, you can update the Personal Meeting ID settings in Zoom so that a unique public link* is created for each class.
*Each class has its own public link. Only you can see this link (from Calendar >> click desired class - public URL). You can use this link to invite anyone to join the class, even if they haven't registered.
To change these settings, log into your Zoom account and head to Personal >> Settings - Meeting: "Use Personal Meeting ID (PMI) when scheduling a meeting" and disable it. as seen below:
Once done, a different public URL will be created for each class. Please note that this change won't apply to any Zoom meetings that have already been created.