Feeding it the following berries cleans it's EVs out. One berry = -10 EVs in the assigned stat.
Whenever the EVs are fully removed in a stat, you'll get a message that "stat won't go any lower." If the pokemon refuses to eat a berry because it's happiness is filled (the berries also make them happy), just feed them some bitter herbs of some sort to bring that back down.Pomeg Berry - Removes HP EVs.
Kelpsy Berry - Removes Attack EVs.
Qualot Berry - Removes Defense EVs.
Tomato Berry - Removes Speed EVs.
Hondew Berry - Removes Special Attack EVs.
Grepa Berry - Removes Special Defense EVs.
Go through and re-train it for EVs, keeping track of the EVs as usual. When you put the pokemon in the PC and then take it out again, it's stats will automatically be adjusted for the new EVs.