The related question section that is generated for each question is quite good. I often find where questions that do relate to the original one. Human feedback can make it even better.
Stage 1. Just show the votes
Stage 2. Feed voting data to spam filters:
- to improve suggestions while asking questions
- to increase connectivity on the site (add top N related questions in the question body, accent on avoiding false positive - do not add unrelated questions)
- to drop unrelated questions from "Related Questions" section (accent on avoiding false negative - do not drop related questions)