I want to max out all EVs except for one, but it keeps shooting up to 255. Any way to avoid this?

[GentleMorningStar]

I have a Snivy with hacked max out EVs (except for Attack). Since it already has a hacked Huge Power ability, max Attack EVs would be way too broken even for my tastes.

So it has 255 of everything and 0 for Attack, but it seems every time it gains an Attack EV by defeating an opponent, it shoots all the way to 255, meaning next time it levels up, the Attack stat skyrockets by 20 points or so?

Is there a way to keep the Attack stat in 0 but everything else in 255?

Only way I can think of is hack in infinite EV-reducing berries but that'd be quite bothersome to implement with every level. Any alternatives?