What are your suggestions to reduce this spam in the future?
In my opinion it is good to use the Karma system to create something decentralized as getting -5 or -10 Karma will lead to restrictions on creating new topics.
Restrictions to create topics is bad idea.
Legit users asking questions will be unable to do so...
New legit users are very important.
I think that reducing the number of boards would make the forum easier to moderate (although I am not a global moderator, I would find very hard to look at dozens of boards for spam)
Moderation is important here to prevent spam links as you said