About a month ago we had a couple of dozen spammers join and not post. Because they didn't post anything, it didn't occur to us that they were spammers but all of them had the same birthday. Then, on that day they all started spamming the board with a whole bunch of spam.
Be careful with non-posting spammers -- they are sleepers waiting for the correct time to strike.
Actually, be careful anything who signs up and doesn't post within a day or two.