I think kicking Hubski off all robots.txt respecting search engines would help against spammers here just for the SEO. Had been suggested by kleinbl00 to keep random mooks who arrive via Google out, but also helps with this. Alternatively, we could take advantage of the nofollow attribute, a la Wikipedia. Not as effective as disallowing all, but still better than nothing. These won't be helpful against "direct spam" though.
I just looked into the guy and his motivations. I truly cannot decipher whether he is attempting SEO keyword spam / backlink spam or attempting to garner clicks from the Hubski userbase. Regardless, neither are well-educated or are going to be a successful endeavor. Hubski's google ranking is far less than other sites and there are sites that are literally made for SEO backlinks and keywords to increase your rank on Google search. The Hubski userbase is full of avid internet users with above average intelligence, above average skepticism, and an above average number of people using AdBlock and Ghostery. I would be very surprised if anyone got any users to fall for scam / spam by posting obvious spam on Hubski. A quick Google search shows that this guy or team of guys is really dedicated to having fake conversations on bitcoin related forums trying to pump up excitement for their obvious scam. Most falls in 2013 when they were "pre-launch." They are pretty good at SEO as it's hard to find a legitimate post about the scaminess of their site without diving headfirst into a pile of fake comments saying it's not a scam.
Just a thought, maybe they are using a "nuke from orbit" approach? Spam forums with both high and low rankings and skeptical users and non skeptical. The ones with a high SEO ranking appear in Google/Bing/DuckDuckGo results making it seem like they are effective, but we don't see the smaller forums they attacked because they have a low page rank. Forums that have people who aren't tech savvy fall for this while tech savvy forum users are mildly annoyed.