Removing users one by one can be done by going to account - people, selecting the toggle icon next to the user's name, and selecting archive user:

To bulk delete multiple users, first create a group called: To delete / users to delete.

Bulk upload a csv of all your users you wish to delete and add them to the delete group. Ensure you select update existing users, not skip, to prevent any duplicate users.

Go to account - people and select the delete group under the group drop down menu:

Next to actions, tick the box to highlight all the users on the page, click the drop down and select archive.

Please note you can only do one page at a time.