An anti-spam technique for protecting search engine ranking is based on mining search engine optimization (SEO) forums. The anti-spam technique collects webpages such as SEO forum posts from a list of suspect spam websites, and extracts suspicious link exchange URLs and corresponding link formation from the collected webpages. A search engine ranking penalty is then applied to the suspicious link exchange URLs. The penalty is at least partially determined by the link information associated with the respective suspicious link exchange URL. To detect more suspicious link exchange URLs, the technique may propagate one or more levels from a seed set of suspicious link exchange URLs generated by mining SEO forums.