That applies to just about everything. We have laws against tax evasion, fraud, murder... but people can (and do) still do these things. Just because a law doesn't or can't physically stop people doing things doesn't mean we shouldn't have the law.
The point is, if it is banned then you will severely constraint the ecosystem. On-off ramps and other intermediators will have to shut down, institutional participants will be forced to withdraw etc etc. Sure no doubt a handful of participants will flaunt the rules and continue to participate, trying to keep the dream alive, but for all practical purposes it'll be dead in the water.