May be some people wanna hack your website or make you angry cause he put order in your shop with no payment and wrong address or spam in your forum, blog ect.
How To Ban IP Address? First you need register in histats.com to get free visitor list, include their IP address.
Then Use the below code, simply past one of the codes in a file named ban.php

For Single IP :

<?php
// For a single user ban
$ban_ip = ‘xxx.xxx.xxx.xxx’; // ip address of the person you want to ban

// DO NOT EDIT BELOW HERE UNLESS YOU KNOW WHAT YOU ARE DOING
$visitor_ip = $_SERVER['REMOTE_ADDR']; // the ip address of the visitor
if($visitor_ip == $ban_ip)
{
die(”<br><br><center><h2>You are banned from this site!<br>Please contact / email Web Administrator admin@yourdomain.com</h2></center>);
}

?>

For Multiple IP :

<?php
// For a multiple user ban
$ban_ip = (”xxx.xxx.xxx.xxx,xxx.xxx.xxx.xxx”); // put ip addresses, separated by commas
// DO NOT EDIT BELOW HERE UNLESS YOU KNOW WHAT YOU ARE DOING
$visitor_ip = $_SERVER['REMOTE_ADDR'];
$ip_list = explode(”,”, $ban_ip);
foreach($ip_list as $ip)
{
if($visitor_ip == $ip)
{
die(”<br><br><center><h2>You are banned from this site!<br>Please contact / email Web Administrator admin@yourdomain.com</h2></center>”);
}
}

?>

Then at the top of index.php files put this code:

require ‘ban.php’;

This script will ban the person(s) from viewing the site.