There's an increased activity recently. In another local small forum of ours we had to block registering for a while as the hosting provider was not happy by massive e-mail notifications of new registrations. Different and more vulnerable engine though. I added a simple question in Bulgarian and it seems to work as the forum is intended for Bulgarians only. Or people knowing Bulgarian.
You can set a limit of post requiring moderator review before a user gets a full posting right. But this puts more load on the admin-mod team.
In a Russian forum they have implemented a plugin: If first post contains a link, the user is deleted automatically.
F1PitRadio @F1PitRadio : MSC, "Sorry guys, there's not more in it"