So simply having the berries lowers EV. Cuz im not getting how they would lower EV if you can't feed them to the pokemon. Except via Pokeblock.
You can feed your Pokemon those berries. This only works in Emerald like ShadowTails said. Each berry reduces 10 EVs of a certain stat. If your Pokemon has over 100 EVs when you start feeding it these berries, the first one will bring its EVs to 100.