Okay, I should've added "permissionless" to the list of desired properties. If you can a priori choose who gets to participate you´d have a point.

> If your country wants to prevent your from participating in an economy, it can

I think that the eternally losing "War on Drugs" should be a lesson that shows this is simply not true. Also, consider how Black Markets are a thing in authoritarian places, not free ones.

So, no, as much as a country can make it difficult for others, they can not outright stop anyone without significant cost to themselves.