Those controls change the value for that flag type in the reviewables score. It increases the chances of a flagged post being hidden by a ‘community hide’, and can also bump those type of flags higher up the review queue.
If your spam posts are being flagged by akismet rather than users, then those scores won’t help. They look like they’re on the default ‘low’ setting already anyway, so you can only make them more sensitive, rather than less.
I wonder if there have been recent changes to akismet that are triggering this new influx of flagged posts?
Edit: You can check out the settings for akismet here: [your-site]/admin/site_settings/category/plugins?filter=plugin%3Adiscourse-akismet
This one may be useful:
skip akismet posts
50
Don’t submit posts to Akismet if a user has posted this many times.