The above answer is incorrect
As stated by Bulbapedia;
>Mega Stones, unlike other held items, cannot be taken via Thief.
Additionally, in this ingame youtube video, Embargo is shown to not stop the opponent's mega evolution, so it leaves the opponent unaffected.
No garuntees on this, but due to the fact that Mega Stones are not affected by fling and thief, and to the best of my knowledge trick and knock off, I would also assume Fling would not affect it.
Additionally if it does, I'd think the Pokemon would not revert to it's original form, since Mega Evolution lasts the duration of the match and I don't know anything which nullifies it.