Spam?

[[vL]Kp]

I wonder if spambots are smart enough to post replies if they can't make new topics.

If they aren't by default, I doubt it would take long for them to be enhanced. That'd be even easier than teaching them to answer the x+y questions proposed above, since it would not involve reading free-form text to find what it needs. All it'd take is the botmaster browsing the site long enough to identify the URL of the add-reply button (you can see it when you mouseover Add Reply) and configure the bot to pick random plausible values for the thread-id parameter passed in.

Another possibility to slow the spam would be to impose a steep replies/time limit on new posters. Something like: if total_posts < 5, that IP address can post at most once per hour. This would not only cripple spam bots (which must then use one open proxy per post in order to produce spam at any significant rate), it'd also curb overly enthusiastic new users who register and proceed to raise several dead topics just because the newbie had something important to say.