A few days ago I started noticing a high volume of registrations from the same domain. For the first couple days I didn’t think much of it. But after 25 registrations from the same domain I knew this was either a bot registering, or a spammer; most likely trying to create backlinks, etc. Follow the instructions below to install a nifty little plugin which will allow you to blacklist domains of your choice within WordPress.
Blacklist/Block Domain on WordPress
In WordPress, go to Plugins > Add New.
Type User Domain Whitelist in the search box and hit “Search Plugins.”
Click on Install Now.
Under Settings click on User Domain Whitelist/Blacklist.
Select blacklist and then choose the domains you want to blacklist. In our example we chose o2.pl.
See example of what happens when a user on that domain tries to register. You can change the error message they receive within the plugin.
You can also download the plugin here: http://wordpress.org/plugins/user-domain-whitelist/