How can I prevent Jira from creating customer accounts?

My Jira Service Management instance is behind a firewall. For security reasons, I don't want to expose the customer portal. Thus, customers can't access the portal. I only want them to communicate by e-mail.

However, when they create their first issue, Jira automatically creates a customer account for them and sends an e-mail with an activation link. That link doesn't work as the customer can't access the portal.

This creates a lot of confusion on the customers end. I don't care if they have accounts or not. I just want a) the communication just happen by mail and b) no e-mail with links to the portal being sent out.

Does anyone have an idea how to achieve this? Any help is greatly appreciated!

Best regards,

Tim