How to block spam-subscribers?

Greetings,
I have noticed a huge influx of new subscribers on one of my WP sites. Pretty awesome. Except that these subscribers all come from Eastern Europe, while my site is actually in Dutch.

On closer look, they fill their profiles with links to stuff I really do not support.

I have now resorted to making signing up impossible, but that kind of kills a large benefit (mails on new messages bringing visitors).

So how do I enable subscribing, while blocking the spammers?