As of latest JIRA, users can't be deleted if they have assigned/reported issues plus if there's any comment made by them.

Assigned or reported issues can be filtered out rather easily and further action can be taken.

However, it is difficult to remove comments by particular user before the user can be deleted. Admin needs to go through all issues that contain comment/s from that user and delete them.

On top of that, searching for comments made by users prove to be really hard as JQL doesn't provide the feature to search issues by comments' author.