Well yeah not all are bots, but a good majority are though. In the case of human spam, most of the time a human just cracks the recaptcha or verification questions then a bot script takes over after the recaptcha/verification question is cracked.
It's easy to tell if it's a bot by looking at the time spent online, if a bot made only one long post and spent a minute or less online it is likely a bot.
It's a bit more difficult to use this concept if the bot made multiple posts since the forums built in flood control takes effect in this regard.
One thing I know is spam is highly annoying and often times a fraud/scam, I'll be damned if I let spammers spam my forum which is why I plan on kicking up the anti-spam features a bit.
Regardless of how much anyone likes porn, the porn bots are the most annoying and disgusting, it's highly inappropriate given that kids or teens usually browse forums.
Also what do you do if a bot registers and decides it's going to post child porn? Personally it's extremely frightening in a way that they can do this and there is essentially no way to track them down due to the spam originating from a botnet or the spammer hiding behind multiple proxies.
Would you be held accountable for the content the bot posts if it is illegal, deleting the posts or banning the user doesn't get rid of everything since pages are usually cached in search engines and still exist in some form on the server itself.