Every system can be gamed, but if it were me and I were looking for a simple filtering solution, I would do something like this ..

Set a policy of X comments required per submission in the last 30 days (not counting last 24 hours) for all submissions, not just "Show HN:" posts.

Meaning, users would need to post X comments before they could post a submission and by not counting the last 24 hours, someone couldn't join, post X comments and immediately post a submission.

It would limit new submission posts to people who are active in the community so they would be more familiar with the policies and etiquette of HN along with gaining an idea of what interests its members.

One thing I noticed recently while going through several of the Show HN submissions was that a lot of the accounts had been created the same day the submission was made.

My guess is HN has become featured on a large number of "Where do I promote/submit my _____?" lists in blogs, social media, etc. to the point that HN is treated like a public bulletin board more than a place to share things with each other in the community.

I love the Show HN section because so many interesting things get posted there but even I have cut back on checking it lately because there are simply too many things posted to check out.

I hope they do something to improve it.