I had a similar problem earlier on one of my forums. That one uses a phpBB3 board, that has the option of having user's accounts manually activated by a moderator. I performed a whois on their IP and if it's not a regular western-European ISP, they're out. I did post a notice for any users who were from outside Europe to contact me directly via email. Not many did
I also have a captcha module and it asks questions, related to the subject of the forum. Plus, and that's a benefit of a non-English target audience, the questions are asked in a different language, one not easily mastered
PS: I manually removed all applicants and banned them by IP. It's a royal PITA, but eventually, after the questionnaire was introduced, the flood disappeared. I did get >250 applications for membership in just 10 days or so, of which 3 (at max) were genuine...