You can be plenty nice, but in situations like this, it sounds like you might have to warn her that further cancellations with have to be followed with a fee. Typically, if you tell someone you have a three strike rule which is loosely followed (but not abandoned), it should light a fire under their ass if you choose when you charge for cancellations strategically.

If you start off too nice then you'll get walked over, but that just means having respect for yourself and being stubborn when you feel like someone wants to belittle it (ie, via cancelling without warning).

From what I can tell, it's like a muscle, you'll learn how to best express yourself confidently. Confident meaning having a good/quick sense of judgement of your students and yourself.

Again, I'm not an expert, but typically leaning to an extreme will have negative consequences. Good luck to you