you can leave it in the pelago, after 24 hours its happiness will start go down quickly (maybe even less hours, i think is based on when the mon reaches max happiness), but you can reset the EVs with berries, and you don't need to have 0 happiness to do this ^_^whats a quick way of getting a pokemon to hate me so i can reset its ev's.
Its a level 100 heatran that had evs prior to being traded to me