Not as of right now.
Actually, it shouldn't even be possible to post a message without a subject, but obviously I've overlooked something in the programming.
As it happens, many of the subject-less posts are spam, so it makes for a nice automatic spam filter, until I have a chance to look at the message. When the messages aren't spam, I usually get around to fixing them within a day or two. When I do, they get bumped back up to the top.
Regards,
Jonathan