Internet registries. ARIN hands out addresses for North America,
a portion of the Caribbean and sub-Saharan Afric (strange, isn't
it?); RIPE handles Europe, the Middle East and parts of Africa and
Asia; LACNIC is Latin America and the rest of the Caribbean; and
APNIC is the Asia Pacific region.
A lot of spam comes from overseas. If you don't care about
getting email from certain countries, you can identify the IP range
those countries use and block everything from that range. It's
drastic, but may be useful. Keep in mind that this would also block
legitimate email, so don't rush into a decision like this.