Can AT&T prove that I am tethering?

So I got an email that basically said “we know you’re tethering, if you don’t stop then we will charge you $40.00 for a tethering plan.” Of course I’m using a jailbreak app, and I’m paying for the unlimited data plan. I assume that they are making this accusation on my immense data usage alone. I def want to continue using my tether without paying for some stupid plan.

I would either like to :
A) get them to admit that they can’t prove I’m tethering.
or
B) get them to back down by making them pretty much admit that they are somewhat spying on me.

Any advice?