Geo-Captcha shows a captcha image only to countries you don't trust. Geo-Blacklists allows you to disable comments for some countries.
geo-captcha-folder to the
You can activate and deactivate the geocaptcha function for comments and registration in the settings. Add the country codes of the countries you don't want to insert a captcha to the whitelist in the plugins settings. Check you log and statistiks to see the countries which cause a lot of spam or who are false positivs.