There is a plugin named Prune Database that can bulk remove topics but you will need to identify the topics in question - there is no facility to identify them by user. And it does take out the whole topic.
Another plugin which I would recommend is our Admin Bar. This allows you to turn on Akismet on forum posts and can, if you have moderation set up - allow all spam to be identified and can be deleted in one, single place. It will also allow you to remove all posts made by the user in question. It is one of my favourite plugins and I use it all the time as I can also reply to each new post without necessarily loading and going to each topic.
But - at the end of the day, prevention is the best cure. We probably get a couple of spam posts each week if not each month and that is largely because we use our own Captcha plugin which you will have just used to write your post. This successfully seems to defeat most automated spam so it is only the dedicated, hand-crafted posts that get through.
Plus if it's truly a user as you say (registered account) in the sp admin users panel, you can delete the user and will have an option to remove or reassign their posts. But here, it's all or nothing on the posts. So would need to be user who has only spam posted.
Mr Papa solution saved my day 🙂
however a bulk delete/move ( because i've a private category used as trash) on a specific list of topics ( witch checkboxes for example ) would be a nice feature!