Hi,
That address should be gone when you refresh the page. In addition, "FA" (FAIL_VANITY when you use a disallowed character in the prefix and FAIL_LENGTH when your prefix is too long) are now replaced with more helpful messages.
That worked, but I had to reload instead of just refresh. Well, either that, or it just took a minute to remove it and I didn't wait long enough.
Adding something about vanity addresses (particularly the maximum length) in the FAQ might help as well.
Thanks!