Facing problem to Clear USB Buffer

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I am sending data to USB (USB HID)like  buf[2]=0x04;

    USBD_HID_SendReport(&hUsbDeviceFS,buf,8);

then I Clear a buffer like buf[2]=0x00;

USBD_HID_SendReport(&hUsbDeviceFS,buf,8);

So it sends zero continuously through USB, So how to clear these buffer and avoid to transfer zero on USB. I am Using STM32F0xx Series controller.