There are sites which have severe problems with persistent trolls -- the kind that, once banned, will come back under pseudonym after pseudonym. A lot of spammers fall under this category. And, yes, I expect that charging money will solve that problem. This may be why some sites have had great success with charging money -- they needed a big hammer to solve a big problem.
But is that a problem HN needs to solve? My impression is that whatever PG is doing to keep out the spammers is working very well. I don't see a lot of complaints about it, even from PG himself. (Not that PG ever seems to complain. He just quietly goes about the business of trying things.) What people seem to be complaining about is a sort of gradual degradation in the quality of comment threads. That's not a problem you can easily solve by banning anyone, unless you really do want to make people flee in terror. ("That LOLcode meme used to be funny, but now it's ten months past expiration! The penalty is death.")
But is that a problem HN needs to solve? My impression is that whatever PG is doing to keep out the spammers is working very well. I don't see a lot of complaints about it, even from PG himself. (Not that PG ever seems to complain. He just quietly goes about the business of trying things.) What people seem to be complaining about is a sort of gradual degradation in the quality of comment threads. That's not a problem you can easily solve by banning anyone, unless you really do want to make people flee in terror. ("That LOLcode meme used to be funny, but now it's ten months past expiration! The penalty is death.")