We could use a topic to keep track of if an artist asks for their work to not be posted. I'd prefer if this not become a discussion thread, and instead just be for listing requests.
November 23, 2012: Cresento/Crenair noted on a picture that reposting requires her permission of any of her work. I sent her a note asking for permission, and received the reply of "Sorry but I'm not okay with it =/". All of her art has been deleted.
Feb 8th, 2013: Crenair has re-okayed her work to be re-uploaded.
Please note that the list of artists in this thread is not exhaustive, as it is Bronibooru policy to abide by "do not post" requests made on the source page (DeviantArt, tumblr or otherwise) unless permission has been explicitly granted to the contrary. Any posts which are noted after the fact to break this policy will be deleted with prejudice.
This list is specifically for artists for whom permission has been sought and explicitly denied, or who have explicitly told Bronibooru staff that they do not want their art posted here.
October ~12, 2012: robynneski - asked his art to not be reposted on any boorus.
May 22, 2012: Hollow(zero): Requested her art to be taken off of Bronibooru.
June ~26 2012: cocatrola - "do not repost" on dA.
November 11th, 2012: whataslendidday - needs permission to repost artwork.
September ??, 2012: deepdarkhana - does not want work reposted anywhere.
December 2012: future-gamefreak/Jeraspat: Appears to request that no images be posted elsewhere, mentioning Bronibooru by name.
January 2012: atelophobix/theprinceofgay: "Don't repost my art" on site, mentions wanting to remove art from Bronibooru by name in one post.
February 14, 2014: zhakrisstol sent an e-mail: "I have recently come across my artwork reposted on the website 'Bronibooru'. I wish to inform you that I do not want my artwork 'featured' there, as such, I am requesting it's removal. Permission was not sought from me by the uploader."
All of the above artists have been added to the banned artists page. Please let me know if their status changes.
Also, mochabean, it is Bronibooru policy to abide by requests by artists to not repost their art unless explicit permission is obtained to the contrary (in which case they will be listed in the "OK to Post" thread and they will not be marked as a banned artist)
March 22, 2014: feebu sent an e-mail: "Hello, I am flattered that my art has been posted on the site Bronibooru, but I would like to request for it to be taken down.
The art was posted without my permission, and I would like for this to be taken down. Along with that, if I could be added to some sort of do not post blacklist, or a 'proof that artist has given permission to post this', that would be splendid."
July 27, 2016: malicieuxx sent an e-mail: "I would like to remove all the artworks with the tag malicieuxx from the site bronibooru since the uploader(s) are not authorized.
The artworks originate from my old deviantArt account malicieuxx.deviantart.com that I deactivated some time ago."
While malicieuxx has not explicitly requested to be on the blacklist, please ensure that you are explicitly authorized to post malicieuxx's art in the future.
August 14, 2015: JackalGiana sent an e-mail: "If I cannot facilitate communication, I would very much appreciate this post be taken down due to widespread theft of my art on other various My Little Pony fanart websites, in which I'm quickly trying to get down.
In future notices, I would appreciate it if my art was not used on your website as well. I'm uncertain how to address this issue since I'm highly unfamiliar to any MLP website because I recently halted my fascination with the fandom due to this widespread misuse of my art. I do not have time to make an account and I don't wish to spam you guys with multiple e-mails regarding one little thing."