If you want to block access to your linux dedicated hosting server for whatever reason, you will need to login to the shell as root. Your server will either have the ipchains or iptables firewall to help accomplish this.
In this example, the attacking IP is 192.168.56.210
If you are using ipchains type: ipchains -A input -j DENY -p all -l -s 192.168.56.210/32
If you are using iptables type: iptables -A INPUT -s 192.168.56.210/32 -j DROP
If you just want to block access to one port from an IP:
iptables -A INPUT -s 192.168.56.210/32 -p tcp -destination-port 23 -j DROP
The above would drop all packets from 192.168.56.210/32 to port 23 (telnet) on your server.