Ment
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There is no core file here, as in NCF. More precisely, there is, but it's symbolic and weighs almost nothing.
All heroes level up. There's not much point in having a limited number if you don't want to play as them; just don't choose them at the start and don't buy them in taverns. Plus, the idea of the mod includes multiplayer, and synchronization is necessary for that mode.
If you still want to disable some heroes, I think Dyrman will provide a brief instruction.