From a software perspective, it isn't the same user, it's no different from making a new account on a different lemmy/kbin instance.
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Also no different from two people creating an account with the same name on two instances.
This is a feature, not a bug. I'm not sure how you know they are the same user, but if they do the same thing that got them blocked before, you can block them again.
I'm not sure how you know they are the same user
Their old account has a blurb providing the new account name. In my case I'm not dealing with a malicious user, just one whose content I don't want to see.
You know they are the same user because mastodon has a protocol for Migration. Basically, you announce on the old instance what your new handle is and confirm from the new instance that this is where you came from. Mastodon then migrates your followers (and blockers). So you can change instances transparent to others.
Of course, this migration is more work than simply creating a fresh account, so i doubt this helps against spammers, but it may help against annoying people.
The technical details will determine what can and can't be done, but from the Mastodon documentation:
https://docs.joinmastodon.org/user/moving/
Moving your account is the same as redirecting your account, but it will also irreversibly force everyone to unfollow your current account and follow your new account, if their software supports the Move activity. Your posts will not be moved, due to technical limitations. There is also a 30 day cooldown period in which you cannot migrate again, so be very careful before using this option!
Depending on if k/m/bin receives a "Move" activity, it may be possible to update user blocklists based on the information in the "Move" activity. However, "Move" activity is generally only sent to existing followers. (I don't know all the details on that) Activities are generally sent to an instance to handle, not individual user accounts, though, so I suspect this might not be as big of a hurdle as it might seem.
Short answer: Maybe. Depends on how they "Moved". It wouldn't be simple to implement, however I don't see anything preventing it in this particular case. You should open an Issue for feature request for it. I recommend including the above piece from the Mastodon documentation, however in your issue.
Thanks for the detailed response!