In old versions of JIRA, a user couldn't be deleted if it had issues reported or assigned by him. In the latest release, if there are comments made to issues by this user as well, it is no longer possible to remove them.
The following message is displayed: Cannot delete user. 'test' has associations in JIRA that cannot be removed automatically.
Change the following and try again:
Issue Comments: 1
Our documentation on deleting users doesn't mention that.