Once a user has created an issue or comment, JIRA will not allow you to delete the user. In fact, deactivating a user is usually a better approach than deleting the user completely. Once the user is deactivated, the user cannot log in to JIRA, and this will not count towards your license count.
Note
You cannot deactivate a user when you are using external user management systems such as LDAP or Crowd from JIRA. You need to do so from the user management system of the source.
Proceed with the following steps to deactivate a user:
Navigate to Administration | User Management | Users.
Click on the Edit link for the user to deactivate.
Uncheck the Active option.
Click on the Update button to deactivate the user.
Deactivated users will not be able to log in to JIRA and will have the (Inactive) option displayed next to their name.