There is a series of books published in the early 20th century. Copyright on at least some of them have expired; they are in the public domain, and are being widely republished.
However, the descendants of the author have trademarked the names of the main characters. This trademark can effectively be renewed forever.
How much control does the trademark give them? I'm interested in whether they can stop the publishing of any derivative works. Would an "(X) character is a legal registered trademark of (Y)" disclaimer head that off? Would the setting and story with the trademarked characters stripped out be permissible?
Of course, there's a specific series of books that I'm interested in, but I wanted the general answer because I think this situation is not unique.