First of all you need to stop more spam attacks.
The captcha you're using on the registration page was beaten a long time ago.
You need a Q&A question that can't be searched.
How to configure Q&A CAPTCHA.
Now, to remove the spam posts, go in to the forum, click the moderator control panel link at the top of the page and then tick the box next to each topic (or choose mark all at the bottom of the page) and choose delete at the bottom of the page.
You can temporarily extend the 'topics per page' in the admin panel post settings to 500 or so to get them all in one go.