Hello:
<<"No. I won't do it, and you can't make me."
Amused disbelief, preferably gentle laughter seems quite appropriate.

Someone who is 30 years old and responding in that manner may have genuine issues well beyond the typical "norm".
<<she doesn't refuse a sensei's request.
Here is the primary issue. Any of us can easily and SHOULD "refuse" if we have a
good cause but politely for goodness sakes. The manner which she choose seems a temper tantrum and literally designed to cause a "power struggle".
Easily handled any number of ways....
"Ok, thank you..." and then ignore her literally as if she was not there would be one possibility.
Public stoning apparently (according to some here) would be perfectly acceptable.

If her stunt was done in a very ~public manner~ then you can make a lesson of it. At the end of that class
not immediately then... so you don't give gas to the fire she created.

Something to the effect,
~...there was a good reminder tonight, for the teachers that we want to talk just briefly about... basic gist:
not embarassing anyone
pointing out
teaching martial arts here...
cannot easily simulate the situations
hope none of you will ever face...
can & WILL create situations
designed to cause ALL of us discomfort...
for some -sparring, for some kata whatever the case everybody has something they won't always love everytime...
everybody has them
A real fight, physical/mental , car accident, boyfriend fight, assault.
We create a small pressure asking everyone to demonstrate in front of the class.
Not accidental, carefully controlled
Done to help you figure out how to handle these small pressures. In time more is
carefully added.
Trust
A different possible approach

not ~hardlined~ to be sure...
Jeff