I've had a statement in my AO3 profile for a while (http://archiveofourown.org/users/mermaid/profile), but I've just updated and expanded it:

If you wish to translate, create a podfic, or make artwork for any of my stories, please contact me first! I'll very likely say yes. However, I will almost certainly say no to anyone wanting to remix or produce a prequel / sequel / companion piece to anything I've written.