You can block any IP address which is trying to reach your server by "iptables".

IP Ban:

iptables -A INPUT -s 192.168.2.1 -j DROP

This makes immediately disconnect on IP.

IP Ban Removal:

iptables -X INPUT -s 192.168.2.1 -j DROP

Both cases, you will use IP you would like to block instead of "192.168.2.1"

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If you know the enemy and know yourself, you need not fear the result of a hundred battles Sun Tzu