transferring from another system to Acymailing. I guess you use an export/import? When you import users in Acymailing that are already in the system and are set to inactive remain inactive after your import.
During import you have these valuable parameters:
Import user as confirmed - Enabled by default. Because of this user don't get a confirmation mail
Overwrite existing user information - Enabled by default. You might want to disable this to keep existing user info.
As for the migration from v5 to v6 the best thing you can do is to run a mass action in v5 to set your blocked users to inactive and afterwards migrate them to v6. In that case they will remain inactive.
Why don't you simply delete users who never want to receive mail from you anymore? By removing them from your system it's impossible to mail them again. You would also be fully GDPR compliant with this as you have no valid reason to keep this data in your system. Chances are these users never com back and if they do they have to double opt-in again. Which is their own choice to do so.