I use a combination of random questions (there is also a mod for that), build in image and email verification, a mod like stopforumspam and a .htaccess ip blacklist of Asian country's.
Next to that I block yahoo email adresses (all of them).
That seems to help.
but most of the spammers are not from asian countrys though.. blocking yahoomail will get you registrations around the world [may be u want only from netherlands].. even with activation from email, verification questions etc still i get some real spammers [not bots] for advtisers for selling .. some from united states, eastern europe, arab , africa etc...