Well behaving proxy servers will send X-Forwarded-For and other extra headers along with the request. Maybe the spammers actually use zombie machines (that don'

Well behaving proxy servers will send X-Forwarded-For and other extra headers along with the request. Maybe the spammers actually use zombie machines (that don't send this), but worth checking at least. Most individual spammers do at least one thing that makes it easy to identify them. Maybe they submit forms 10x faster than any human? Maybe no referer? A unique user agent?