Gone are the days when you need to go through the REST API or pay for an app just to delete users or create users. I have always felt the need to help people as much as I can and doing so I have created a free app that helps do just that, which can be downloaded for free. You can install the app called BulkOps app from the Atlassian Marketplace and perform bulk deletion of jira users or customer users.
Just to give you a background, on Jira there are two user types that exist.
In order to use this app for bulk deletion, all you need to do is prepare a CSV file having just 2 columns. The id column to capture the accountId of your users and a displayname column just to show you who the users are.
Delete Jira users
To delete Jira users, simply prepare a CSV file of your user list. You can get this from your admin hub by navigating to https://your-instance.atlassian.net/admin under the user tab click on the export users button to the top right side.
Once you download the file, open it and delete other columns leaving only the "id" and "name" columns then save the file. That way when uploading this file with the app, it can know the right user to delete
Delete customer users
Similarly, if you need to delete customer users, navigate to https://your-instance.atlassian.net/admin/jira-service-desk/portal-only-customers and click on the export customers button on the top right side.
Once you download the file, open it and delete other columns leaving only the "username" and "full_name" columns then save the file. This will be the format of the file you will upload using the app.
Perform a bulk delete after you've structured your file by launching the app - navigate to the users > bulk delete user and upload the file and that's it.
If formatted correctly, the app will delete the users you have on the file.