They should disable the button for the duration of the transformation effect (the first 3-4 seconds that it takes you to actually shapeshift). SOOO many times I've accidently hit the button twice thinking that lag caused it to not go through when really it was just the animation taking a second to play out.
Honestly, though, they need to remove the cost and duration from the transformation ultimate entirely, and just make it a toggle on/off that lasts however long you want to stay shifted for. We're already giving up an ultimate slot and all of our class/weapon abilities (really any non-werewolf abilities) just to use werewolf form, it's asinine to give it such a short duration.