Right now, the only way you can do that is in the search: add a "-" before a keyword (and yes, I mean keyword, since the search doesn't only look for the tags) and it will be filtered out in the results. Example:
www.deviantart.com/browse/all/… vs.
www.deviantart.com/browse/all/…This is usefut, but not perfect, as you can see, even with something that isn't as synonymous as flying islands and floating islands.