A Newsvine author or seeder has the ability to block any† nation for any reason. If a single viner comments from a nation on one of your publications you can block the entire nation from all of your publications with a single click.
It is a very blunt instrument.
Now why on Earth would someone use this capability?
To Block or ...
There are some obvious situations where a nation block makes sense.
Egregious CoH Violations
The most obvious is probably the intended usage. If participants in a nation routinely and egregiously violate the CoH on your articles or seeds then that nation is harming the experience for everyone. As an author or seeder it would make sense to block that nation from your publications. However, a few viners do not represent an entire nation. So before impacting all members of the nation an author might want to first engage the nation's admins. If the admins show no interest in taking control of the situation then blocking the nation is a sensible course of action.
If a nation blocks you then it does make good sense to block it from your work.
The reasoning is simple. If a nation's admins do not want your participation then they should not want to engage on your published works. A reciprocal block seems productive and generally a good thing for Newsvine. This is one way for those who cannot engage without counterproductive slap-fighting to agree to avoid each other.
... Not to Block
For the most part the nation block function is simply another toy for rogue author / seeders to play with.
Some authors block a nation in response to its opposing views. They hide behind the block function when the counterarguments are too good for them to handle. Comments that support the spirit of the CoH should never be justification for blocking. Authors who cannot deal with CoH-compliant topical disagreement should probably refrain from publishing in a public forum. In particular the practice of writing a closing 'screw you all' comment in a nation followed by a BLOCK is particularly cowardly.
Then there are those who seem to enjoy imposing negativity on other viners. Some apparently get a 'rush' by clicking that BLOCK button and of course proclaiming said blocking to the entire community.
Nation blocks seem appropriate as a last resort to combat aggressive, CoH-violating comments or to exclude nations that have blocked you.
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Are there other reasons for blocking and, of course, other examples of when not to block? One can also choose to disagree with what I have offered here but your nation might get blocked if your argument is too good for my ego to handle.
† - 'The Spirit and the Purpose' nation cannot be blocked.