I'm using my own fork of the Registration Blocker plugin, which reduces the spammer registration volume to a manageable number. Don't expect it to be a silver bullet, though. It takes some training (i.e. adding the domains and/or mail addresses and/or usernames abused by the spammers), and even then fighting spammer registrations remains a continuous effort.
The current spike is to be expected, since apparently many admins are on vacation over the holidays.