Historically speaking, the prohibition of anything has not really worked out so well for those who are doing the prohibiting. Just because something is not allowed doesn’t mean that someone, somewhere isn’t going to find a way to do it anyway. And the overthrow of a ruler-however powerful- is a really good incentive to break the rules for those individuals who are after power or simply ambitious enough to try it. That being said, I’m not sure that I could come up with the “best” way to keep someone or someone’s from trying to overthrow Ozma. If you have enough people and enough motivation, overthrowing someone can happen with brute force when it comes right down to it. The bottom line is: if someone wants to overthrow Ozma that badly, they’re going to find a way to do it. It just may take a bit more planning without the use of magic. I’m not sure I would have done anything differently, though. Maybe she just extended her stay in power by outlawing magic.
To know you are ignorant is the beginning of wisdom.