If you want to block an IP to access your linux server, 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 192.168.45.45
If you are using ipchains type: ipchains -A input -j DENY -p all -l -s 192.168.45.45/32
If you are using iptables type: iptables -A INPUT -s 192.168.45.45/32 -j DROP
If you just want to block access to one port from an IP:
iptables -A INPUT -s 192.168.45.45/32 -p tcp -destination-port 80 -j DROP
The above would drop all packets from 192.168.45.45/32 to port 80 (HTTP) on your server.
