When idling with the bike up on the centerstand, it's normal for the rear wheel to spin by a small amount, especially when you first start the bike and the engine is cold. After the bike has warmed up for a few minutes (or after a ride, while the engine is still warm) the rear wheel shouldn't spin more than a very, very small amount.
If the rear tire still spins even after being warmed up, the idle is likely set too high and should be adjusted. This has to be balanced against the need to keep the engine idle sufficiently high to keep from stalling, especially on carburated engines. Given a choice between a stalling scooter and one with the rear wheel spinning slightly, it's probably better to put up with a spinning rear wheel.