MP Spam Be Gone is the simplest, most effective Spam blocker.
Absolutely not. Set it and forget it (and it will be easy to forget, when the spam stops).
If Version 3.0 does not correct this problem, please check your theme templates. In the comments.php page, you should see a function - comment_form(). This function was added with WordPress 3.0 and is required for my plugin to hook into the comment form.