@coolbean @yassie_j @whitequark at some point, you can't. @FediTips is right about that.
I mean, mail is federated... how many users would your service get if you blocked gmail.com? Same idea.
However, if there is too many random upsets, I don't think it will mean "mastodon keeps decentralised". 99% of people don't and never will care, so it's more likely 99% of people simply will stay on twitter etc.
Making it like twitter but less shit (open, no algo, etc) means a chance at replacement...