They absolutely can ban encryption if they want. Just like they can ban mathematics. What makes you think it's impossible? It wouldn't immediately eliminate

They absolutely can ban encryption if they want. Just like they can ban mathematics. What makes you think it's impossible? It wouldn't immediately eliminate all encryption, but it'd be easy to go after any corporations operating in the US and either force them to follow the ban or throw their execs in jail. After that, they could put up a Great Firewall and block downloads from foreign servers. They can use the Great Firewall to snoop on internet traffic, doing deep packet inspection, and look for unrecognized traffic, VPNs, encrypted emails, any markers of common unauthorized encryption protocols, etc. They don't have to catch every single person, they just have to catch some and then make an example out of them. You're probably thinking that this is just too extreme. Has everyone forgotten what life was like in East Germany under the Stasi? That lasted until the late 1980s! Now of course, you can debate how unlikely this scenario is to play out, but I'm just pointing out that it isn't impossible, not at all, and we've seen a society much like this in the very recent past.