People who haven't earned 1 merit shouldn't be allowed to post link, that will counter somewhat.
The forum is not built for it. It is built for freedom and theymos does not want to have newbies jail again AFAIK.
Limiting newbie participation is very harmful for a community. Newbie jail will never return: I consider the newbie-jail period to have been extremely damaging to the forum. When barriers to participation are too high, then the best people often just won't go to the trouble of joining, and the people who are willing to jump through the hoops are often people who aren't good for the community: people with nothing better to do, scammers, get-rick-quickers, etc. Having a permanent newbie jail policy would improve things a lot in the short-term, but would end up being a fatal poison to the community.
The low signal-to-noise is a real issue which seriously annoys me and is often on my mind. But as you mention, fixing it non-destructively is difficult.