Thank to both of you for the comments. Yep, spam is here to stay, at least for the foreseeable future. The board I'm putting up is on a site that will shortly go live and be somewhat high profile, so I expect the bots to find it.
Smartys, yep, I don't allow guests to post--and that plugin for visual confirmation sounds like a wonderful way for me to limit spambot registration. (Even that can be cracked. Some bots will forward that graphic file to porn sites, which pretend to require the confirmation--so port site visitors become part of the graphic-cracking program. Doh!)
Smartys, do you know what the plugin is? If not, I'll start lookin'. Hopefully it'll be easy to find--the open source blogging software I use doesn't have a central directory of plugins, so my experience says I might have my work cut out for me, to find this.
Thanks again, all!