Sounds like you have windows media player to auto synchronise. There is an option to switch it off - in the sync screen click the arrow below sync, navigate to the device then select set up sync and unselect the sync automatically check box.
When you want to put more music on, you go to the sync screen whilst the device is attached and drag the music you want to the panel on the right ahand side and then start sync.
Hope that helps
Shaz