I would argue that barring brand new accounts from posting is less likely to unfairly inhibit speech than manually banning spam accounts. I think both practices are fair game, but the first is an automated standard that would apply to all users equally, while the second is a subjective determination. I trust boobs and AOU, and whoever else they would appoint to be janitors, and generally I'm in favor of the practice of manually banning spammers, but with that approach you always run the risk of different interpretations of what spam is. And if their job is to ban shills too it's even worse. I know, I know, it would be 'only the obvious, low-effort ones', but it's a total judgement call.