There are many reasons to block an IP address from accessing your sever. Most are security reasons, for example, to prevent Site Scrapers and Spammers. Otherwise, they might just be for personal reasons; for example, you may run a forum with an annoying member - blocking their account wouldn't stop them from signing up with a different email address; blocking their IP address would mean they couldn't access your site at all from their machine.
To block an IP address from viewing websites hosted on your 123-reg hosting package, please do the following:
Login to your 123-reg control panel.
Scroll down to the web hosting section.
Select the relevant domain name from the drop down box and click on the Manage button.
Click on the IP Address Blocking option in the Web Tools section.
Enter the IP address you wish to block into the Configure IP blocking field and click on the Create button.