Geo-Captcha shows a captcha image only to countries you don't trust. Geo-Blacklists allows you to disable comments for some countries.
!!! IMPORTANT !!! If you have used the old Geo-Captcha plugin please completly uninstall it (the form is on the main page of the plugin) and remove the plugins files. Geo-Captcha & Geo-Blacklist ist fully based on the old Geo-Captcha plugin but is not compatible with it and uses new names for options and databases! !!! IMPORTANT !!!
There are a lot of ways to prevent spam. One that is still pretty often used is to let your readers type a code from a captcha image to a textbox. Geo-Captcha uses the same technic but you can define a whitelist with countries. People from this countries will not be forced to enter the captcha code. You can also define a blacklist which prevents people from some countries from posting any comments.
The location of the visitors is checked using the GeoLite Country-Library from MaxMind (http://www.maxmind.com). The captcha image is generated using SecurImage from drew010 (http://www.phpcaptcha.org).