delete a portal user?

I recently deleted a portal user. Or at least I thought I did. I was still able to login on the portal as the user and they still had access to projects. How can this be? When I logged out and logged in again as the admin/portal owner, the user account had been restored. We have employees and/or clients come & go all the time and must be able to delete them without having to worry about them logging in. Thanks.