It's pretty obvious the usa version of Code Geass is a royal fuckup.
Her name is spelled カレン (Karen) and not カッレン (Karren/Kallen), yet somehow the idiots romanized it as Kallen when they brought it over here. It's clearly wrong, and danbooru has been known to use romanizations which are correct though not official; or just plain weird (toosaka/ooji and many more examples). So what's the deal here with the alias getting denied, Kallen is completely idiotic.
This is not the "USA version", Bandai Entertainment did not make it up. It's been like that since the beginning, long before it was licensed. The Karen/Kallen thing has been debated before here and elsewhere, and the consensus is that it's an intentional thing - the name goes both ways to reflect her her dual Britannian-Japanese identity. (See post #89882)
It's the name used on the ANN encyclopedia, generally Danbooru's stated authority file on non-Japanese names. Likewise, both names are in the wikipedia entry, with Kallen being entered first and associated with the Stadtfeld surname. And it's also used on the official site: http://geass.jp/first/world_03_ex.html?pid=chara_04.html in plain English. As this is not a situation of a straight up Japanese name with no caveats, the usual rule of "straight, correct romanization" does not apply. The non-Japanese half of her identity changes the dynamic.
What you *can* do is suggest an alias for kouzuki_karen to kallen_statdfeld (we're keeping the latter as the tag that displays). kouzuki_karen is indeed the Japanese half of her identity. It's just not the one used here, or much of anywhere else.
In short, chill out and quit it with the self-righteous rage at getting your alias denied. If you think a tag should be changed, you can state it as such without asking "why the hell" we're siding with "royal fuckups" and "completely idiotic" decisions. That's more likely to get you a negative record than to get you your way.