Bug for comments in the blog

Meal, if I delete a comment in the blog it disappears, but the blog says I answered to the comment and counts to delete that, so my last blog entry now has 6 “Geisterkommentare” although I have deleted only three. That brings me directly to my next question: Can I determine users who are not allowed to submit comments thereby not unnecessarily flaming and provoked? Or directly lock out, that would be even better. Greeting, Kami