[Digital Jedi]

Sometimes the best to way to mass delete threads is to just pull up the relevant threads in a search window (either user by user, or with the new posts option) or go forum by forum, and check all the spam threads/posts in the moderator column. Delete or move as you see fit. Another way is to use the prune/delete threads option in the Admin CP. Really, it's just a matter of what is more convenient for you. When the number of posts gets particularly large, there's just no getting around the more tedious aspects of picking out the spam posts.

Another option I often use is in the spam poster's account in the Admin CP. There's an option there to delete all his posts. You should run this, though, before deleting his account if you intend to do so, as his posts will all revert to Guest and they're won't be a good way of finding those posts anymore. This can seem tedious, too. But it's a little more precise and a bit quicker than eyeballing the posts yourself.