If you need to interrupt the download process you can do it any moment by pressing the button "Stop" in the application toolbar (main menu item Project\Stop download; shortcut Ctrl+F2).
BackStreet Browser does not interrupt the download process irreversibly, instead it stores a savepoint of this download. If in the future you return to the project and press the button "Start" in the application toolbar (main menu item Project\Start download, shortcut F9), the download will be restarted from that very point where it was previously interrupted.