It doesn't work that way. When using async consumers the messages are delivered as fast as the onMessage method completes. If you want to consume one and only one message then use a sync receive call.
For an async consumer the prefetch allows the broker to buffer up work on the client instead of firing one at a time so you can generally get better proformance, in your case as the async onMessage call completes an ack is sent back to the broker an the next message is sent to the client.