With this WangGuard Add-on, you can allow or disallow domains for signup on your website. It is compatible with WordPress, BuddyPress and WooCommerce. WordPress Multisite has its own allow/disallow, so its not implementet.
WangGuard plugin version 1.6+ is needed
If you are a website that you want to allow to signup only cetains domain, with this add_on you can do it. Or maybe, you want to block certain domain that are not in WangGuard Domain list.