On the Connect app, there’s an option that users can enable to make user notes global. This applies a user tag to all accounts with the same username across instances. Any accounts that get a copy of the tag has an added * to denote that it wasn’t originally tagged; as a reminder since some people may simply have the same username on different instances.

I prefer to tag accounts over blocking because I think it’s beneficial to upvote pushback or correction replies, which you wouldn’t see by blocking. But some people have way too many alts and it’s rather tedious to tag each one. Granted, I also have many accounts, but that’s mainly for testing purposes.