If you have more than 200 hundred nonactive JIRA On-Demand users after large project – of course, it’s taking a lot of time to get rid of manually all the useless information like comments, assigns, issues and user profiles.
Moreover, it’s not possible to remove users in JIRA Cloud if they left comments in a issue or if they have any issues assigned to them.
First of all create a ticket for Cloud support team and ask then to move all the comments of those users to a existing user.
After that, if those user do not have any issues assigned to them you can manually remove those trough the User Interface.
Provided solution, unfortunately, is the only way to delete users in JIRA On-Demand yet.