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Block a host (iptables firewall)
Thanks to lowendtalk.com and Alex_Liquidhost for this:
iptables -A INPUT -s x.x.x.x -j DROP
where x.x.x.x is the IP address
I haven't tried it but it seems fairly straight forward and should work.
Of course, please be careful using the iptables command since it can be used to block all connections to your server.