Well, it is a dangerous subject to incorporate something that can't really be locked down to a logic or science. However, you can usually weed out the ones who wish to abuse the power by making them come up with the hows, whys, and whats of their magic. By doing that, you can force them to come up with reasonable limits based on their own descriptions of their 'special skills.'
Another way of doing it is to get together with AoK and come up with the hard limits of what Kohanians can and can't do on and off their planet. In fact, you could restrict magic to only those born on Kohana.
I know it's not the most scientific thing, but you could basically create artificial works of magic by saying 'nanomachines did it.' Making it so that any spell someone could cast could at best duplicate what technology can do is a good option. My power could summon a spirit from a nearby river to help me catch fish, but you could get basically the same result from a fish finder. On Kohana his powers might be a bit exaggerated, but he can't take that unbelievable power with him. Off the planet, he's a glorified medicine man, communicating with spirits like a native American shaman would (with no mega-blatant supernatural affects).
Well, that's my comment. I love figuring out little plans and plots like this, and I'd be glad to help work any hypothetical situations out that you might have trouble with.