In order to delete a User from a Security Role you need to login as an administrator and click on Security roles in the menu.
Click  beside the Security Role. This way the users who are assigned to this role will be displayed : in order to delete one or more users from the role click the red X beside the name. An Alert to require the cancellation confirmation will appear. Click ok.
By leaving checked the send Notification box, a notification email will be sent to the user informing him of the role cancellation.