i know how to do it at script level but how to do it at apache level?

Mod_GeoIP from MaxMind can do that: http://www.maxmind.com/app/mod_geoip

I haven't ever used it since I've never found the need to block an entire country from viewing my sites. YMMV!

Put this in .htaccess

Order Allow,Deny
Deny from .ca
Deny from .mx
Allow from all

Replace canada or mexico with another unimportant country of your choice :)

Wow, that's pure brilliance! Thanks