Rhomboid - 7:30 am on Nov 15, 2006 (gmt 0)
If they are determined to mess with your site then this will have no effect. For one thing, these IP-to-country mappings aren't always accurate, but more importantly it's easy enough to use a proxy somewhere in another country (or Tor) to bypass these kinds of blocks. If I were you I'd focus my efforts on blocking the specific behavior (e.g. implement capchas if they are flooding posts) rather than the country.