WHMCS is doing this already.
reference:
http://docs.whmcs.com/Security/Ban_Control
Banning Email Domains
With WHMCS it is possible to ban email domains from signing up. This is useful if you want to block customers signing up using free email accounts.
To enable this feature, simply go to Setup > Manage Banned Emails. You will then see a list of all the currently banned email domains and the number of times a customer has attempted to signup using them.
To add a new banned email domain, click the Add tab at the top of the page and then enter the email domain you wish to ban, for example "hotmail.com".
Auto Ban Control
WHMCS by default blocks any users IP who attempts to login to the admin area with a valid username and incorrect password three times or more. The length of this ban by default is 15 minutes and is designed to prevent hackers being able to endlessly try different password combinations in order to gain access to your admin area. You can however alter the length of this ban to increase or decrease it by going to Setup > General Settings > Other tab > Exceeding Invalid Login Attempts Ban Time