Currently there is no way to disable an internal users login in sakai. This is a common flag one can set on most sytems and is useful in a number of system administration senarios.
It would entail new fields on user: disabled (boolean), dateDisabled (Date), disabledBy (String UserId)
Users that are disabled should:
1) Not be able to login to sakai
2) Should not appear on lists of site users etc
3) Should not be able to be added to sites