If MBC is set to automatically login using a certain user profile, and that user is disabled, the login should fail and not allow other users to be selected.
For example, last night our daughter got in trouble and was not allowed to watch TV. In her room, MBC is set to automatically login to her user profile. I disabled her profile so she could not watch TV, but when she woke-up her computer to try to watch TV, the MBC login screen came up and allowed her to select from other users.
I don't use any of the other clients, but this may be something to look at with them as well.
Thanks!