All "Captchas" are not enough today. But what do you want to do? In some countries, people get money for write spam in blogs, boards and so on.
I do not know about a god spam detector and to build some one, is not easy.
For spam bots are exists some nice "tricks", like: rename field names from "email" to a hash-strings, but for blind person, you must use a label-tag and a good spam-bot can understand this.
XenForo shouldn't worry about an anti spam system. It's someone elses job. XenForo should only provide support to third party anti spam solutions. IMHO.
Do you see any bots on this site so far? They're probably trying to figure out where the register button *is* - only humans will think to click the pulldown register form.