Technically yes, but generally you don't delete users as it's a pain in the neck. As Andris says, just remove them from all groups and you'll get their seat back. I'd recommend changing their name to show they've left, removing them from all roles as well as groups and setting their email to a dummy.
Depending on the version of Jira, you can "inactivate" users (fairly recent to the most current versions have this option,) which leaves all their groups and history intact, but makes the user unable to log in and frees up the license. If you're on On-Demand, you simply uncheck all of their application access and this will have the same effect as inactivating the user on the download versions of Jira.
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