You can use the wastegate, a blow-off valve, or a restrictor to limit the amount of boost (backpressure in the air intake side of the engine). The wastegate controls how much the turbo spins and thus how much air it moves. The blow-off valve leaks the pressurized air (think of letting pressure off of a tire) into the atmosphere. The conventional use for a blow off is to relieve the compressed air during engine vacuum. The restrictor works by limiting how much air can enter the turbo. This means that the wastegate will be unaffected because it will depend on how much boost exists in the intake manifold. A retricted turbo will need to work harder for the same amount of boost (or be more efficient).