Benny-
You could block one or even a few domains leveraging DNN's built-in RegEx validation feature.  However, this would not be an effective method for blocking more than a few domains.
 
I'd recommend that you check into the nice Dynamic Registration module from DataSprings.  You can use the module's "Custom SQL script validation" feature to validate against own DB table of blacklisted domains.  Beyond that, though, you can really get fancy with your site regsitration.
 
Dynamic Registration also lets you create custom registration forms, check available usernames without a postback (using AJAX), process payment at time of registration (PayPal and/or AuthorizeNet), send custom emails including file attachments and much more.  Some people forgo the bells and whistles and instead use DR to make signups simpler than the stock DNN user reg module.  All in all, lots of features packed into a very nice module.
 
Hope that helps-
-mamlin

Thank you for yor help!