If you're site is running on a popular forum software, robot-only inputs and timestamp analysis would eliminate most of your problems. Spammers are probably no

If you're site is running on a popular forum software, robot-only inputs and timestamp analysis would eliminate most of your problems. Spammers are probably not targeting your website in particular, rather the software your forum is run on. If you add atypical anti-spam measures you'll separate yourself from others using the same platform, defeating the typical phpbb or vbulletin bot which probably accounts for most of your spam.