here's a solution for you
remove flying mounts on PVP REALMS ONLY!!
if you can have lower level guards on pvp realms only, you can remove flying mounts on pvp realms only.
as it would seem that it's only ever people on pvp realms that want them removed because they want world pvp, i say give it to them!
edit: you dont even have to remove flying mounts, you could just disable flying - good luck questing in deepholm!