You can choose to ban users from using certain types of email addresses during registration. You can also ban users using certain IP addresses from your site.
To do either of those, make your pointer click on the User icon (the 3rd icon from the bottom) in the Admin Panel and click Ban rules.
Click Add new to add a new ban rule.
Enter a reason in Ban name / Reason field.
Enter either an IP rule or an E-mail rule.
To replace unknown characters use * (the asterisk) symbol.
When you're finished, click Add new ban rule to save your changes.