If poster is new (# of posts is zero), they shouldn’t open a new topic.
For legitimate new posters, they should get a page that tells them to post to an existing thread, after which they can open a new topic. A spam bot attempting to open a new topic won’t read that page.
I lot of the spam would stop if the admin was more involved. Simple daily reviews of the postings would reveal these problems and then action could be taken - delete the spam and ban the ‘members’ posting the junk.
This action was taken before after a few pranksters posted porn and hate images here. Can’t be that hard to do on a regular basis…