I love the idea but I think forcing your standards to other users who might be not as well educated and fluent with English is not the right idea.

The feedback left on users profile is "Part of the Spammer Blacklist: this user has made at least 50 replies that are not up to forum standards."

The problem here is that you are setting the forum standard, the better option might have been leaving a feedback like "Part of the Spammer Blacklist: this user has made at least 50 replies that are not up to my standards. This is for my own reference and nothing to do with forum officially."

Also tagging something like "This user is part the top 1000 worst spammers of all time." shows you mean to insult the user instead of making a list to help yourself in future. Who gave you the permission to call someone a spammer just because you didn't like his English? Would be be fine if someone calls you a spammer because your Spanish is not on point?

If you are actually intending to make the list for reference in future then leaving a small and non-insulting feedback might have been sufficient. If you want to insult users because of their language, I think you are not presenting yourself in the best manner and need to do better.