Spam is definitely a concern. I'm using 1.2 and a CAPTCHA plugin which prevents most automated spam, but I still get some spam, some of which is human-generated, but some of which still could be automated (hard to tell). If Flux had a "Moderate user's first post" feature like some other forums, I imagine I could catch 99+% of spam that way before it hit the board, but there's no such feature. For now I'm stuck with finding spam after it happens, which is a extra pain because my forum emails recent posts to a mailing list every day (homegrown script) and I might not catch the spam in time, and the email recipients are really sensitive to spam, naturally. I'd like to upgrade to the latest 1.3 SVN version of Flux but I don't think there's even a CAPTCHA for that, so that would be a step backwards.
So in my mind, neither the latest release version (1.2.something) or the development version (1.3 SVN) is suitable if spam is a concern. Once 1.3 gets to the RC (Release Candidate) stage then we can expect a CAPTCHA plugin, but I don't think I've seen any plans for a Moderate First Post plugin. And there's no telling when 1.3 RC will be out anyway. The initial release date was supposed to be around June so it's about four months late and ticking, and the developers haven't given any ETA's for when it will actually be released.
I wish I had a more uplifting answer, but I'm just trying to be honest. Maybe two months from now we'll have an RC version, CAPTCHA, and Moderate First Post... or maybe we'll have none of those things.