>ITT we brainstorm about how to prevent unwanted influxes of newcomers that change the culture of imageboards.
You're fifteen years too late, and according to some people, earlier than that. Most of the damage was done long ago, even 4chan is rotting, not only evident from merely looking at the place any time in the last five years but reflected in the stats too.
That said, it becomes much easier on:
- a self-hosted imageboard where you can hack in features
- a board with an actual topic (if you have the luxury of banning politics, or all politics outside a containment thread, this does absolute wonders for gatekeeping)
- a board with an actual culture (surprisingly rare after so much dilution from 4chan/8chan sites)
When I say a board you can hack in features to, you can even gatekeep with custom captchas (e.g. name this molecule, which class owns the means of production, select all the female anime girls), so long as you're willing to suffer the high barrier of entry - it's pretty common to hear people calling imageboards a dying medium and not without good reason. So if you're going to block off all users who aren't already intimately familiar with your topic, it's going to filter out, maybe, 20 commercial CP spammers and wandering schizo /pol/troons and 2 actually curious people who just can't answer the question yet.