I've thought about this too and it looked like a tough problem to solve, but it isn't.
Why do people do that?
They usually do it for Ask HN posts and well you can easily discriminate for them. Further if anyone tries to spoof the system by adding a URL there then we can parse it to see if there is any text there according to the stupidity filter. So, essentially it will still work.
Comments
I honestly think this is the way to go, especially if you figure out a good damper on frivolous commentary.
There's always the stupidity corpus that mr. Graham used to talk about...
http://stupidfilter.org/wiki/index.php?n=Main.Status
That would prevent people from using throwaway accounts, which a lot of people seem to do. And they are not spammers.
I've thought about this too and it looked like a tough problem to solve, but it isn't.
Why do people do that?
They usually do it for Ask HN posts and well you can easily discriminate for them. Further if anyone tries to spoof the system by adding a URL there then we can parse it to see if there is any text there according to the stupidity filter. So, essentially it will still work.