The difference between blocking and purging is that a blocked account is still residing on the instance (even more so for user blocks instead of admin blocks). As the data is still there, it opens possibility for legal trouble. Full purges by instance moderation action is different.
The scope of my proposal is not to help curate and filtering content for instances, but to enable widespread coordinated action against content which was already created and is harmful without any room for interpretation.
approach could be any IP
That is a whole other issue. Scrapers and request spammers, not content spamming accounts, can be combated with already existing technology at the reverse proxy server. There are big crowdsourced IP banlists which are continuously updated.
I think subscribable block-lists for instance admins
This would be the easiest solution. But I think it will lead away from individualism of instances, as all decisions are automatically applied, instead of only a manually reviewed subset.
Your idea with applying the mod action after a threshold of other instances did it too is something to consider. It will keep individualism but also creates "avalanches" once enough instances in the network agreed on a decision, which might be even better for propagating swift action against general harmful content.
