You can also have the spell on the wand and require n mana to be put into it for activation (argument), now warrior type goblins won't be able to use it either.

There is no "correct way" with OOP: There are as many ways as you've got the energy to think about and all of them are leaky somewhere. You can still write maintainable code with it, but making it sound clear and obvious where what should go is just wrong. It heavily depends on a multitude of factors and can get nauseatingly complex at times