There is no way for the superuser to change the password for a subuser. All you can do is delete the user and then re-add them afterwards. I was pleased to see that a post is not also get deleted when it's author is removed from the system. But, since there is some sort of relational juju going on between the posts and authors databases, the remaining posts suddenly become orphans. At the very least, this makes for the tiresome task of going through a bunch of posts and reassigning an author for them. At worst there isn't presently much you can do about identifying an author whose posts you want to keep but who is no longer an active MT user. Probably the right thing to do is assign a "deleted" date property to each author and then modify the permissions check accordingly.