I read somewhere that certain Berries will reduce certain EVs by 10 points each time you eat it, so you can start all over. It was mentioned for the Emerald version of the game, but I'm pretty sure it holds true for Diamond/Pearl now.
I have a few of these berries, and they do say it 'Lowers the Pokemon's base (something) stat'.Quote:
Pomeg Berry lowers HP EV by 10
Kelpsy Berry lowers Attack EV by 10
Qualot Berry lowers Defense EV by 10
Hondew Berry lowers Special Attack EV by 10
Grepa Berry lowers Special Defense EV by 10
Tamato Berry lowers Speed EV by 10
So if you wanna completely reverse the EV training, gotta start planting (and fertilizing) those berries. I used fertilizer once, and I got 6 berries from one plant, 4 berries from the plant next to it and 3 berries from the one next to that. I only plant the things and water them once whenever I do plant them, so getting 6 from just one plant was pretty big.