Denver boot

n. locking device attached to the wheel of a car to immobilize it (used especially for cars parked illegally or until the owner pays parking fines that accumulated)

A wheel clamp (American English: Denver boot, wheel boot, or boot) is a device that is designed to prevent vehicles from moving. In its most common form, it consists of a clamp which surrounds a vehicle wheel and is designed to prevent removal of both itself and the wheel. It is often used for security purposes, such as preventing a trailer or caravan from being towed away by a thief, or to stop one's own car from being driven away by a thief.
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a wheel block that stops a car from moving Greg was drunk, so the police put a Denver boot on his car.