If this is some kind of tournament or something like this, then, don't set limit to coins buying, but the quantity to use of premium items, for example, 50max or something like this. This way maybe you balance the war.
Aigrind don't lives because this person spends hundreds dollars in one day. I mean, they don't live from 10 person spending 1000$/year, but 5000 spending 50$/year. If you allow to 1 person to smash the game experience to others, then you can loose this 5000 base players.
As Crystana wrote, if you abuse of P2W, you loose base players, and then, Hassn don't needs to spend this hundreds dollars on scrolls, and all goes to sh#t. It's a domioe effect.
Don't know, as mentioned I'm not sure if I understand exactly the problem (but I think is something like you can use some powerfull premium item using your VISA card to win the competitive scene of the game), I'm playing since 2 days ago. I just wanted to feel part of the community and throw my humble opinion