Only site owners can remove users from a site.

1.  Click on Account > Site Admin > Users
2.  Find the user(s) that you want to delete.
3.  Click on the delete icon next to the user's name.
4.  Confirm by clicking Yes.

This will remove them from all workspaces they are a part of, and stop them from logging into the site, but will keep all the work they have added to the workspaces.
Deleting a user will also free up the User Licence that was allocated, so a new person can be invited in their place.

If a deleted user requires access back at a later point, just invite them to an appropriate workspace. 
The invite email will contain a link that will allow them to re-register on the site, where they can set a new password for their profile of which will not contain their previous workspace membership - if you want them back in the same workspaces you'll need to invite them to those workspaces.

Alternatively, if you require deleting a lot of users from your site who no longer require any access then this could be time-consuming.
Therefore our support team can assist by performing a bulk delete of your users from the site using our admin tool.
If you wish to use this facility then please raise a new support ticket with the following information, refer to this article below.
How to contact support and view your tickets

* Your Kahootz site name and website URL address of where those users reside.
* A full list of the user's containing their email address & UID that you would like removed/deleted.

Note: the bulk delete request can only be raised and approved by the site owners.