AFAIK, this subforum was created to do keep other subforums free from alt currency clutter.
So if it gets closed, it's assumed that the clutter would just go elsewhere.
sadly, this
If there's no subforum for this and there are some people with moderation rights, you can simply ban them for a few days for being off-topic. Permanently if this person continues to post something like that.
Which is a lot of work for mods. Some people will simply register again and continue to slip alt-coin nonsense into other threads.
You could say the same thing about "off topic" forum. It exists to keep the off topic BS there.