You can't. JIRA won't let you destroy that audit information.
Just make the user inactive so that they don't count towards your licence usage (and I tend to remove them from all groups and roles, change their name to something to indicate they've left, and set their email address to a null mailbox)
If you want to delete, one approach would be to bulk delete the comments and all information associated to this user in the database, however, this is not recommended nor supported, and it would destroy audit information
The best approach would be making the user inactive as Nic mentioned.
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