I know there are various plugins for adding capchas to the forms - specifically comments forms that are prone to getting spammed, but I was wondering if anyone has written a spam filter that checks for, and blocks certain strings from the main comment fields - eg, if the post contains 'http://' or 'porn' or 'drugs' etc etc we could automatically reject those submissions?

I'm also thinking it could be worth being able to block the submission by checking the country of the IP address - eg, only allow submissions from the countries you want.

Or for comments forms, you could just hide the form altogether for countries other than the ones you specify