According to our documentation on Deleting users:

To delete a user:

Locate the user in the User browser (see Viewing Users above) and click the Delete link in the Operations column.
The confirmation screen that follows will summarise any involvement of that user in the system by showing current issues assigned to and reported by that user, etc. These connections between the user and other parts of the system may prevent the deletion of that user.
Take any actions required to disassociate the user with JIRA. These may include:
Reassigning any issues assigned to the user.
Bulk-editing the issues created by the user and change the 'Reporter' to someone else. You'll need the 'Modify Reporter' permission to do this. You will also need to allow editing of closed issues if some of the issues the user created are closed and you do not wish to reopen them.
Changing the owner of shared dashboards owned by the user. See Managing Shared Dashboards.
Changing the project lead for any projects that the user is a lead of.
If there are no issues assigned to, or reported by the user, the confirmation screen will display a Delete button; click this to proceed with the deletion.

Please Note:

The filters and dashboards of a user will be deleted when the user is deleted, regardless of whether the filters or dashboards are shared with other users.
Any numbers of issues which have been reported by or assigned to the user you are attempting to delete, are respectively hyperlinked to a list of the individual issues (in the Issue Navigator).

However, if a user made comments to issues, it cannot be deleted. JIRA will display the following message:

Cannot delete user 'xxx has associations in JIRA that cannot be removed automatically.

Change the following and try again:

• Issue Comments: 10

This needs to be included in the documentation.