Unfortunately, wheel-clamping companies are allowed to clamp cars if they are employed to do so, even if they only have tiny invisible signs. It is an offence to damage or remove a clamp which has been put on your car, so if you chained it to your car's axle and asked for payment for it to be removed, then you would be in danger of being prosecuted for theft of the clamp, or demanding money with menaces, or attempting to extract money without authority (I do not know the proper legal phrase).
So you would be right (morally), but you would not be within your rights (legally).