Once accounts reach a certain reputation threshold, the account can no longer be destroyed by the moderator.
However, that restriction seems to be circumventable by first suspending the user (which sets the user rep to 1)? At least in the moderator dialogue box, after a high-rep user is suspended, the "destroy" radio button which usually is greyed out becomes available for selection again.
Note that I didn't test whether the button would actually work: I don't want to destroy someone's account just "in the name of science" ;-p
!