In T10215#197817, @epriestley wrote:

From what we've seen on this install, the "printer fax support" spammers are humans willing to go to significant lengths to overcome access barriers (they fill out Captchas, register and link GitHub/Google accounts, validate email addresses, successfully navigate workflow changes, originate from different remote addresses, and take actions slowly), so I suspect no automated system designed to deter bots will be effective against them. My best guess is that they're being recruited through Mechanical Turk or some similar system.

Ah, thanks for the tip, did not thought about that. Will do the trick for me, for sure.