Service Desk: Customers waste JIRA licenses

Question

While testing JIRA Service Desk I have a licensing problem. For each customer in the service desk a JIRA account is automatically created. This is not wanted, because we only use a JIRA license for 25 users but have much more customers. I did assume to use create JIRA users only for Agents/Developers/Admins. Did I select a wrong setting or is a license upgrade required?

Service Desk 2.3.6

JIRA 6.3.15

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I just upgrade to JIRA 6.4 / service desk 2.5. By now, service desk customers to not consume a license.

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Up to now it does not matter if I create the customer account (in Service Desk => People => Customers) or if he user creates it by himself.

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You say it only depends on the Version of Service Desk? I've guessed about upgrading jira, but would like to avoid this by now. 
Can you give a hint, which might be the bad setting? Service Desk has only very few settings, I have no glue which one could be wrong.