Is it illegal to lock the wheel of a car and demand money for unlocking it because the car was parked in an area of reserved use by the person placing the lock in the car?
It might be illegal, depending on where you are.
If it is known to the person who owns the vehicle occupying your space prior to doing so that wheelclamping may be the result, then the common law doctrine of volenti non fit injuria ("to a willing party, harm is not done") would lead to a conclusion that one who knowingly places themselves in a situation where harm may befall them is not entitled to bring a claim of tort against the other party (in the United States assumption of risk is a similar doctrine, but doesn't apply here because wheelclamping is an intentional act). In your situation, this means that the owner occupying your space cannot bring a case in tort against you.
While it is generally applied to harm to people, there is precedent (albeit in British case law) for the doctrine to be applied to vehicles, and specifically, to the situation you describe - see Arthur v Anker and Vine v London Borough of Waltham Forest.
While cases from other jurisdictions are not binding, I have not been able to find any similar cases in the United States, and so it is possible that such cases will be persuasive. Note that this is highly jurisdiction-specific, and there may be laws in your state that make it unlawful - at least one high-profile case involving a McDonalds and its parking lot operator cites California law authorising only law enforcement to impound vehicles, and considering wheelclamping such an act. However, I have not been able to find the record of a judgement on this matter.
It is also likely that if the payment you request is excessive, the owner would be able to seek relief, in the form of reducing the payment owed to a reasonable amount.
However, if the parking space was not marked in some way to signal that it was reserved for use, then the owner of the vehicle may be entitled to seek injunctive relief and damages from a court. This would be on the basis of, if you only wheelclamped the car and declined to remove the wheelclamp upon the owner's request, the tort of detinue.
But, if you attempted to request payment from them, then as Dale mentioned in his answer, you have committed extortion (wheelclamping is actually listed as a crime of extortion).
As stated in In California, if a baseball lands in my yard, is it legally mine?; denying possession to the rightful owner of their property is the tort of detinue. Demanding payment for its return is the crime of extortion.
By parking on someone else's land you have committed the tort of trespass and the owner would be entitled to damages up until the time they put the lock on the car; after that, they have removed your power to end the trespass. Damages would have to be real, for example, if the owner could demonstrate that because you parked there they had to park and pay somewhere else. Even then, your trespass does not excuse their detinue two wrongs do not make a right, see Bike locked to mine: am I allowed to cut through the lock on my own?.
A government body doing so in accordance with a valid law is entirely legal.
If you have agreed that this should happen by entering a contract with the person; this is entirely legal too. Terms and conditions posted at the entrance to a carpark where a person entering can read them are generally an enforceable contract; by entering and parking you have demonstrated sufficient intent to be bound.