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 Now we'll just have to worry about the IP list becoming outdated and blocking legit signups