At level 100 you can't EV train. A pokemon will only get EVs if it gets experience points. Since a level 100 pokemon doesn't get experience points any more, they don't get EV either. So if you're using the EV reducing berries, you have until they level to 100 to get your pokemon just the way you like.
By the way, the EV reducing berries have been around since Emerald. If you have over 100 EV points in a stat, using the appropriate berry will take it down to just 100, every other berry after that will remove 10 ev points. (In other words, 11 is the maximum number of berries needed to reduce a pokemon's EV in a stat.)