those enabling them by responding to them should stop.

This seems to be the main problem, but my recent attempts to discourage those who make high-quality contributions to hopeless or malignant conversations didn't stir much enthusiasm, so it'd probably take a lot of effort to change this.

(A specific suggestion I have is to establish a community norm of downvoting those participating in hopeless conversations, even if their contributions are high-quality.)

Please crack down earlier, harder, and more often.

This is something new for LW, in fact this appears to be the first time when a non-Eliezer moderator stepped forward to implement this measure (in this case prompted by Eliezer's recent statement that deleting posts by a chronically downvoted user who doesn't stop is to be considered a general policy).