I often have situations where I need to prevent access to a specific user account, sometimes temporarily, sometimes permanently.
More often than not, this is for security reasons.
For accounts that need to be permanently disabled, it would be easy to say "just delete them" but there's often a lot of content associated with that account that would need transferring to other accounts. This may not be appropriate for many reasons.
My way around this at present is to change the password but a much more effective method IMHO would be to have a simple enabled/disabled checkbox.