I think a lot of people might already know this but having only recently found it myself. I think it's a good opportunity to share it with everyone! If someone has posted your art on deviantart without your permission, there's a really simple way of having taken down instead of filing a report (which, from experience is completely ineffective). All you have to do is fill out this form, it's a DMCA form which means as the copyright owner of the art you can elect to have it taken down, by law.
I didn't find out about filing DMCAs on deviantart until I googled it (silly I know) but they have long existed on tumblr and I've also seen one on weheartit.com. Filing DMCAs seem to be a lot more effective, the art is taken down in a couple of days. And in the case with tumblr all reblogs of the post in question are also deleted, along