You can block specific geographic locations by visiting the block location page that you get to by clicking the Edit Profile tab. Simply click on the states and/or countries you want to block and hit Save. Guys visiting the site from those locations won't be able to view your profile.
You can whitelist specific guys who otherwise would be blocked because of your location blocking settings. Here's how it works. Let's say you have blocked all guys from the state of Ohio. When you visit the profile of a guy from Ohio, you will be given the options to 1) Block, 2) Leave Feedback and 3) Whitelist. If you click on "whitelist", the guy will be unblocked but your location blocking of Ohio stays active and all other guys from Ohio remain blocked. You can later block a guy you have whitelisted by clicking the block link on his page.
You can also permanently block guys if you want to unblock a location. When you click "block" you permanently block the member. Even if you unblock the location, like the state of Ohio, the guy will remain blocked.