You can block the domains used for registration by wildcard. Before I block a domain I'll first go to ACP >> Users & Groups tab >> prune users link on the left. For the email text input use
*@example.com and hit submit. This will give you list of active accounts with that email address. You'll be able to determine if there is legitimate users using that domain. If it's all spam accounts I'll prune them at this point.
If you wish to proceed with banning the domain click the ban emails link on the left. Add
*@example.com and click submit.
One thing I'm not sure about is how this would affect current user, if they already registered I do not know if that bans them but if you purged the accounts that won't make a difference,