The Sovereign Citizen crowd are indeed nuts. When I was a law student I spent some time when learning how to do legal research practicing by thoroughly debunking Sovereign Citizen claims on Usenet, and continued to do that after I had mastered legal research and no longer needed extra outside practice. As a result I've seen pretty much all of their arguments.

Their inability to understand basic logic and the breadth and depth of their ignorance of history was astounding. It was only exceeded by their inability to understand even the simplest aspects of how the legal system works--they often for example cited documents as being decisions of a court when in fact the document was a brief from the losing side stating the argument that the court had found to be incorrect.

So what I find amusing is that in the particular situation you brought up they have out-reasoned you. :-) It is one of the few situations where they are actually taking a rational approach.

If someone actually believes that they are truly sovereign and not subject to the license plate rules it is in fact rational for them to put fake plates on their cars when driving through an area where the police will harass them over a lack of plates. Even if they were right and that harassment is not actually legal it is still a pain in the ass to deal with and a fake plate might prevent it.

Hey, one of the latest imports from the US to France.

We had the case of two sovereign citizens stopped by the police who was dumbfounded when these people were constantly saying that they "do not contract with the enterprise French republic" (the sentence does not make grammatical sense in French).

After what they were brought in for interrogation (and I think a psychiatric evaluation). The trial is planned for April because their house burned down and they need to apply for the state subsidiaries (the last part is a joke :))