I have had numerous attacks on my site where scripts ( I guess) register automatically and fill up my database while over-using available resources.. even though I have to approve users for final membership I still have them registering in plenty like 100 dummy registrations a day.. How can I stop that.

Same applies to guestbook...

I havn't tried this yet and not sure how it works but check this out:

extensions.joomla.org/component/option,com_mtree/task,viewlink/link_id,116/Itemid,35/

Let me know how it works.

There might more components out there like this... not sure