To block unauthorized connection or IP address from accessing a LInex computer, you can use a firewall.
The most common firewall tool used in linux distributions is iptables, although some distributions might also user firewalled. Here's a basic guide using iptables:
1. Identify the IP address to block.
2. Setup rules with iptables
sudo iptables - A INPUT - s<IP_Address> -j DROP
3. Save the iptables rules.
4. Testing.