Not everyone who shows up on your website does so with noble intentions. Banning people’s IPs to keep them away from messing with sites is a standard practice for webmasters. The IP Filter plugin for WordPress allows you to add that functionality to your website easily.
IP Filter lets you create white/back lists of IPs for your site. It supports wildcards too, so you can block large sets of IPs in fewer steps. You do have the option to show a message to those you have filtered. This plugin can be used to create completely private communities or keep trolls from accessing your site.
IP Filter is quite flexible and does what it promises well. Just make sure you don’t add your own IPs to the list.