How to fight against spammers ?

Hello,

I have regularly people subscribing on my Elgg install to spam. I've first tried to fight against that with Capcha, and now with questions I thought would be good to stop spammers (like "which is the first name of the french president :)). But it's not enough. My Elgg install is not old, not more than two or three monthes. What can I do ? Is there a plugin to moderate subscription with mandatory admin activation ?

My Elgg install is here : http://com.savs-epilepsies.fr

Thank you