There is an easy solution. Rebrand Alliance common as Azothi. Humans and every Alliance race speaks Azothi now. Common is something everyone can speak, but it is locked behind a questchain that has to be done on every character you want to speak cross faction with, and is something you can toggle on and off in Options.
The trivial inconvenience of the quest chain will cut down on the trolls. The toggle lets people engage with it on their own terms. And repeat offenders could, in theory, have Common removed from their account.