There are a few ways you can do this:
1. You can use route. Syntax looks like this:
/sbin/route add -host *IP TO BLOCK HERE* gw 127.0.0.1
2. You can use iptables / ipchains:
/usr/local/bin/iptables -I INPUT -s *IP TO BLOCK HERE* -j DROP
/sbin/ipchains -I input -s *IP TO BLOCk HERE* -j DENY
This will block all access to your server.
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