These frustrations are understandable. On the whole one could say that saying "Implement #ActivityPub and federate" raises wrong expectations.
And that it is more accurate to say that AS/AP et al gives you foundation stones and building blocks that constitute a framework for federated app development. Unfortunately there's been a lot of ad-hoc interoperability and protocol decay. The trend is not to maturity but chaos.
If fighting spam, esp. on the #Fediverse still interests you, I may point you to @emacsen and @csddumi who are about to launch a project to tackle the problem.
Funnily enough, I already discussed RBL technology a long time ago, and there's even an issue on it.
Making an RBL is fairly easy. I prototyped one in an afternoon using rbldns. The harder part is determining what hosts (or IPs) go into it, and continual maintenance.
I've also written up (somewhere) a set of RBL status IPs (ala the 127.0.0 block used by traditional RBLs) that reflect more fediverse-type issues.
AP + HTTP Signatures take care of many of the issues that email has, but not all, though most of the attacks I've identified in the Fediverse are still theoretical and not seen in the wild.
I've been hesitant to publish them because it would be too easy to pick up and cause havoc.
Anyway, I welcome experience/ideas and especially help.
@humanetech You put it in good words. There's a lot of discussion of "Just ActivityPub it" as a way to "grow the ecosystem". The reality is details, scope, and purpose of federation is more than "just do it", and the expectation that everything federates smoothly with everything just because ActivityPub exists sets everyone up for failure.