How would a governmental Bitcoin ban work in practice?
You may be thinking about this the wrong way.

Most countries have a ban on cars driving above a speed limit. However this is not enforced by using technical means that prevent cars driving over the limit. 

These rules are enforced by doing unpleasant things to people who are detected as having broken the rules.

You may go on to suggest that rules need technical means of detection. But remember that detection need not be perfect. If the consequence of detection is that you lose your job and you life is ruined, few people will take the risk.