Normally if you want to block all requests to and from an IP address, iptables is an ideal solution. A rule similar to this one would be handy

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/sbin/iptables -A INPUT -s 1.2.3.4 -j DROP

What if you want to block that IP without using iptables, route command would do the job

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/sbin/route add -host 1.2.3.4 reject