For starters, consider whoo and Balupton's WP-Deadbolt plugin http://www.village-idiot.org/archives/2007/01/10/wp-deadbolt/ to prevent people from registering with domains you don't like. Then update the plugin's array. Here are the domains I no longer permit to create an account:
$badmails = array(
'126.com',
'163.com',
'2youm.info',
'alejandrinarynes.info',
'altaairconditioner.info',
'blah.org',
'bumpymail.com',
'cashette.com',
'centermail.com',
'centermail.net',
'chazears.biz',
'discardmail.com',
'dodgeit.com',
'e4ward.com',
'email.net',
'email.ru',
'emailias.com',
'fakeinformation.com',
'forex-broker-news.com',
'front14.org',
'gawab.com',
'ghosttexter.de',
'ibuzzmachine.com',
'info-webs.org',
'jetable.net',
'kasmail.com',
'lawyer.com',
'link2mail.net',
'lynellvantrump.info',
'mail.ru',
'mailexpire.com',
'mailinator.com',
'mailmoat.com',
'messagebeamer.de',
'mcn.com',
'mtmlcore.info',
'mytrashmail.com',
'nervmich.net',
'netmails.net',
'netzidiot.de',
'newpostcash.org',
'nurfuerspam.de',
'privacy.net',
'punkass.com',
'sheryllvasaure.info',
'sneakemail.com',
'sofort-mail.de',
'sogetthis.com',
'spam.la',
'spambob.com',
'spambob.net',
'spambob.org',
'spamex.com',
'spamgourmet.com',
'spamhole.com',
'spaminator.de',
'spammer.com',
'spammotel.com',
'spamtrail.com',
'tashiavanhamme.info',
'trash-mail.de',
'trashmail.net',
'wanettavais.info',
'web.de',
'wuzup.net',
'yandex.ru',
'yoricksite.biz'
);