Yes, you can lower your EV's. There are certain berries specifically assigned for each stat which can lower your EV's. In concern to your scenario, you want a Hondew Berry to lower your SpAtk EVs. The first berry will reduce your SpAtk EVs to 100 (if the SpAtk EVs are above 100) then every Hondew Berry afterwards will take away 10 EV points.
You would want to fined some Hondew berries and perhaps plant them in Poke Pelago to produce more if one will not suffice. After you reduced enough EV points, you can EV train your Atk stat.
As a side note for you, if you have remaining EVs in other stats, here are the berries that do the exact same thing:
Pomeg = HP
Kelpsy = Atk
Qualot = Def
Grepa = SpDef
Tomato = Spd
Hope I helped! :)
