You can inactivate one. That removes it from the license number but leaves the history of the issues intact. You won't be able to logon with the account any more. That is the standard procedure for retiring accounts no longer in use. Since JIRA is a relational database it keeps the user's database key, not their ID in the issues so deleting a user will cause issues when the id is referenced, even in comments. Besides, you may want to know that the guy from HR who left 2 years ago created the issue some day.
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