But, if you don't like it, then it is relatively easy to start your own community site. Something like Lemmy or kbin might be suitable for you. Let us know how it goes - and whether you solve the spam problem.
They are "cheapest and easiest" because they're the ones that currently exist. Any other approach would require work on the part of the site owners. If you want them to change that then you're going to have to present a case as to why it's in their interest, not why it's in yours.
Nobody is opposed to you setting up a competing forum with different spam-filtering qualities.
But, if you don't like it, then it is relatively easy to start your own community site. Something like Lemmy or kbin might be suitable for you. Let us know how it goes - and whether you solve the spam problem.