Unfortunately the only way to block someone is through their Profile page, but I am not sure how this person was able to hide their Profile, as I can find no Miiverse option or setting that will allow this.
And the only online reference I could find about this mentions that the Profile might have been moderated by one of the Miiverse Administrators, so if that is true then likely the person has been removed or their Miiverse account was deleted and can no longer follow you.
But perhaps you should call Nintendo's 1-800-255-3700 (6 am ~ 7 pm Pacific, everyday) Customer Service number, or use their webform to contact them, and they will be able to tell you if a person can hide their Profile and if there is any other way for you to block them.
And if you do contact Nintendo, could you come back here and tell us what they might have said, as there will be likely others that may need to know what to do in this situation, thanks.