Okay, so first you will have to get rid of the EVs. This can be done by feeding it 11 berries per stat. If a Pokemon has over 100 EVs in a stat, the EV-lowering berry will reduce it to 100 no matter how high it is, and after that, each berry will decrease the stat by 10 EVs after that. Because of this, feeding a Pokemon 11 of a stat decreasing berry will always result in 0 EVs in that stat.
And don't worry, it's not too late. I think once you get to Level 100 there's nothing you can do, but up until then, you should be fine.
After EV training your Pokemon (I'm assuming you understand this part), you can level it up as usual. A Pokemon can only receive 510 EVs total, and no more than 255 per stat (most people stick with 252, so none are wasted). So, as a result, battling random wild Pokemon can not change the EVs you have already received. I would recommend doing whatever it takes to get your Pokemon to level up as quickly as possible (after EV training of course).
First, put whichever Pokemon you want levelled up in the Day Care. Each step you take is an experience point, so you need to walk around as much as possible. Next, go to the Gear Station in Nimbasa City. Then tape two pennies together over the B button, and two more pennies over the down arrow. Your little person will run around and around and around. Next, plug in your DS charger. Leave the DS on all night, and when you wake up in the morning, you will have crazy strong Pokemon :) And the Pokemon in your party will also like you more, and their happiness will increase ^.^ You're welcome.
Oh, one more thing- if you don't actually understand EV training, here's a guide from Serebii: http://www.serebii.net/games/evs.shtml