yes, disabling the function whereby an answer lands an email in your inbox ought to be enough. And apparently that was disabled long ago. Apart from that, this is supposed to be an archive of useful material, so there doesn't seem any point in preventing people from adding to it.
The closing thread option has proved handy in CB when the thread has gotten out of hand in some way. I do agree that it is open to abuse though. Some new posters have mistakenly closed their threads in other sections.