Yeah but you can outlaw specific uses of it. Not saying I agree with it but crypto can definitely be made illegal.
To be clear, we don't get to choose which parts of math we ban and which parts we don't.
Banning or preventing people from using math only empowers other countries that develop it on their own.
I think what you're thinking is the ability to convert fiat to crypto. If you got rid of that, it would just move more underground and into the black money markets.