I just blogged some thoughts about “Who is welcome to my Inbox” and am curious how you think about it:
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I really like “#Seppo, the first person tooter” – could put that as the Seppo category description.
“whitelist/blacklist” were deprecated in our circles for “allowlist/blocklist”. 
I generally share your concern and cut of various interlocutors. When it comes to blocking spam, I’m also a bit worried about the unintended consequences and side-effects: for example, someone could block someone else out of spite, only to unblock days later, but the block could reflect and cascade to other instances and snowball into full cancellation. Just imagining a worst-case scenario…
Knocking
I find the concept attractive, but 24h and 3 days sound a bit long in Fediverse terms. I don’t think there’s a FEP yet for this.