Hmm. I think having to actually go into the comments section to tag something as spam has a few benefits. By creating a slight barrier to make posts as such, it helps to prevent abuse. At the same time, it also keeps someone from accidentally mislabeling a post as spam because if they go into the comments section or the poster's profile, they may see that the person just made a lame post. Personally, I think a good alternative would be that someone wouldn't be able to post until they fill a certain percentage of their hub wheel by being a thoughtful, positive contributing commentor. Not only will this prevent a large number of spam accounts, but also discourage trollish behavior.
Overall, I like this idea, but I feel like it would have disparate impact on well-meaning dumb people, people with a weak grasp of english, and probably some other well-meaning people I'm not immediately thinking of. Maybe a timeout on the post restriction could fix that problem, but the other issue that might arise from this would be comment spamming.