I would be tempted to email anyone who has signed up and not posted to ask them if they are having trouble - good customer service! - and if their email addresses bounce, or they respond with junk then delete their accounts.
I +1 mikey_w because on a reasonable size discussion forum (1,000+ posts a day) I have worked with, I do indeed find that spam bots sign up as sleepers. Sorry if that ruins the joke ;)
You don't mention what software you use, but keep an eye on how many IPs you ban, as with phpbb I have seen pages run slow because of a massive ip ban list, it gets cached in a good set up, but I have noticed requests where 90% of the build time was checking for banned ip.
I am never entirely comfortable banning by country, especially since learning as much as 40% of dodgy traffic comes from the USA (on the discussion forum I work with, YMMV)
Instead of just checking by country, check out Project Honeypot they have an api you can use to find known spammers by ip, they will return a score and often a comment like "known comment spammer" that way you can ban bad ips rather than black marking entire countries.
Project Honeypot gave birth to CloudFlare which is a CDN that sits in front of your site and provides speed and security upgrades; it is well worth a look at it or similar solutions (I don't work for CF but am a fan). Using either of these methods you can stop them even getting to your site. You don't say if you pay for data transferred but CF's bandwidth is free so it helps with that too.