Sometimes you might want to block connects to your server by IP address. E.g. when someone tries to attack your system. This can be done in several different ways, e.g. with iptables (firewall) rules or by setting up a reject route. Using iptables sometimes causes conflicts with installed firewalls so that I will use the route command here. The command is:
/sbin/route add -host 192.168.0.123 reject
Replace 192.168.0.123 with the IP address that you want to block and execute the command as root user on the shell.