For fan fiction I have quite often seen disclaimers saying:
"The characters, universe, and events are all copyrighted creations of [insert author's name here] and [instert publishing company here]. This was made for fun and not for profit. Please don't sue." (That last part, I'm given to understand, is sort of optional.)
So, for JMS, it would be something like:
"Ivanova, StarFuries, and EarthForce are all copyrighted creations of J. Michael Straczynski, Babylonian Productions, and Warner Bros. This was made for fun and not for profit."
Even so, however, I've seen this mostly on Whedon fan fic, and Whedon's famously more encouraging of fan creations. In fact at one point he even encouraged bootlegging copies. So tread carefully, and make friends with lawyers.