It seems to me the spam always is entering the first forum 'group' on the whole list. e.g. 'Piratelogs Shortwave'.
Couldn't you make some kind of dummy group and place it at first position in the list.
I hope the bots don't understand the difference between for instance 'dummy group' and 'Piratelogs Shortwave'. The spam is placed in the top most group and we can read the logs without being overwhelmed with spam.
Concerning the SPAM
Moderator: Moderators group
It's of course possible, but most post are from dirty persons. I've been several time on line with a spam-poster and after deleting the post he put another post on the board. Beside that the spam post are not alway frequent. However there are spam bots signing in but don't post anything. First cleaning and protecting the database would be nice. But I guess there will be always spam with an int. forum like this one.