Of course you can.
If i wanted to do that, i'd parse a complete front site and do some regexp magic to find something of a "forum help" subforum, and then autopost this kind of topic.
But like i said, most of bots are pretty dumb, eg simplistic programming. Bots target sites with no validation of registration - with captcha things get very complicated. I'm confident this was a human, and that's easy to check. Forum admins can look up his latest login date before time of this post. If they differ in a few seconds, cause that's what it would take for a program to parse, find, and post, with all the comm latency in between, then it's a bot. Normally, if that range is above 10 seconds, then it's human. Again, from a programmer's viewpoint, you can deliberately delay the bot from posting - to make it look like a "slow" human - but that wouldn't be too effective on open forums where bots can literally spam boards with hundreds of topics in a few seconds.
In any case, with captcha and email verification on registration, and enforcement of a minimal period (think it's 30 seconds here) between new posts from one user, you're pretty much guarded from spambots.
On the other hand, spamming humans come with volume - you have a popular board, it's going to attract their attention. There is a way to counter that : disable signature / avatar / any kind of user customized visuals for new people that don't have 10 posts. With no obscured link in the signature, spammer needs to clearly write his spam in the post - like visit www. shitstore. com, and then nobody is going to click on that link. They are coming from a perspective that you won't see right through them on the first glance - it's a normal post in a normal topic, it's a new user, and he has some link in his signature - i might as well check it out. No link - no checking out - no spam.