We have been suffering a bit of spam recently.
I increased the aggressiveness of the spam filter for 24 hours and whilst it blocked the spam during that period, it also blocked some genuine topic replies.
So for now I have turned it back down as I would rather have spam that can be deleted later, then genuine conversation delayed until approved.
I will have a tinker with the registration process to see if that can help.
Sorry for any inconvenience this may have caused.