Question How can I permanently delete hundreads of spam posts?

Yes, in principle you're right. There are to many steps for this option to do that if you have so many spam. But you can also prevent the problem by delete permanently all user posts during the ban. In the frontend "User -> tab New Ban" is this option available.
If you want, then create a topic for your sugestion here in this category please.

Yes, you are right. I had already tried the option "permanently delete all posts of this user" when creating a new ban. But, this operation resulted in 500 error because this spam user has more than 30,000 posts, so Kunena cannot move them to trash either delete them permanently. At the end, I have to delete those spam posts MANUALLY, in many steps, with only about 200 posts for each single batch.

How many hours this will cost me to completely clear those spam messages? You can imagine that.

You might fix this with phpmyadmin:
www.kunena.org/forum/k-3-0-general-quest...na-mass-delete-users

You don't need manipulations in the database, and this guide is already more than 5 years old. Go in the backend to Kunena -> Trash and purge the items.