I've noticed two things about all the bots: they all have IPs originating in China, and they're all registering with e-mail addresses from the same domain-- @163.com. Is there an option anywhere in the admin panel to ban a domain from being used during registration? If so, that may be enough to confuse the bots and stop them from relentlessly creating new accounts to spam us with.

No at the moment this is not possible. But banning user names and email addresses will be included in Version 2.1.2. The SPAM issue is a huge problem at the moment, the 2.1.2 Release will be delayed because of that. There will be some more spam protection options included.