Technically, Uni should be suing Six Flags. Superman & Batman are honorary Avengers and, according to some very loose interpretations hear, that's violating the contract.
...huh? Batman and Superman don't even exist in the same universe as the Avengers, as they're owned by two different companies. And you can't sue Six Flags for using characters they have a licensing deal to use, just as Disney can't wrestle Marvel from Universal just because they feel like it.