As I mentioned earlier, I constantly check the Tag Board for illegitimate messages. I remove them whenever I can, but I have to check every day at least once.
Meanwhile, if you do see a spam message, just ignore it. Do not, repeat not, click on any links you may see in it. You run the risk of downloading a malicious file - virus, trojan, whatever.
I am looking for other solutions to the Tag Board. If all else fails, we may have to revert to the traditional comments system, which is less immediate than our tag board, but it is fully protected.