While I agree that it's a good idea to remove their firearms, it would be interesting to see how a (real) legal challenge worked out.
As I understand it, one of the requirements for possessing a firearm is to be a "fit and proper person".
If that's the legal basis that the police are using, it does raise questions about how you determine that someone is UNFIT to possess a gun.
If the test is that you posted some shit online which makes you appear to be a sovereign citizen, where are the limits?
Anyone at some point can be critical of the government, I've done it myself on many occasions. Do we want rights to be removed in that case?
As I said, I don't disagree with the action, I just hope that the police are using substantial proof before acting.