How do you ban cocaine? How do you ban special parts in guns that make them automatic? How do you enforce any law that's not practical to enforce?

You just make it a felony with a penalty of decades of imprisonment. Then, all the businesses stop doing it and you selectively enforce the law in order to make an example out of people. The idea is never to enforce it fully.

If encryption is a felony, your average business will stop using it, which is pretty much the desired effect. They don't care so much about your personal website or data store using encryption. They'll just make it a fringe technique.