How about just letting the account get created and then disabling the account? I believe Moodle will only allow one account to be associated with an email address, subsequent attempts to create new Moodle accounts using the same email address should not work.
I have a Moodle 2.2 with captcha enabled for email-based self-registration and still get spam accounts created every couple of hours on average. I disable the accounts when I find them, most every day. So this costs the spammers a captcha attempt that somebody would have had to do, but it seems there are no shortage of people somewhere to pass the captcha test. At this point I assume that my captcha test is being repurposed by a spammer to have people falsely believe they may be unsubscribing from getting junk emails.
Thanks Rick, I have already voted for and am watching the issue since earlier this year. This would be a welcome enhancement to the user filter advanced settings.
From my comment in the "Spammers Using Self Registration" thread, I may have another idea to prevent spam accounts (copy & pasted from there):
I think others have suggested something along the same lines of your suggestions. I like to keep the country set to United States, to make it easier for my students. The spammers on my system have email accounts from aol.com, and hotmail.com. But so do some of my valid users.
All these spammers don't make it into any course, which is why I want to filter on this condition. Ideally, I would prefer a filter that states "Delete users not in any course after xx days."
I agree with the filter (and have voted for it) until such time have you looked at geoblocking?
Not sure if it would suit your situation but having just cleared out 1000+ spammers yesterday then 100 new today we've just blocked anyone from outside our country from accessing our site.
To do so, rather than using the inbuilt IP blocker we went with using the .htaccess file (less hassle I believe). Now we'll just have to worry about the IP list becoming outdated and blocking legit signups