On the far left side of the mix view, make sure the Instr. button is highlighted. This hides or shows the Instruments box immediately to the right. You should see all the instruments used listed under the Instruments box. Right click(or use the dropdown arrow) the ones you want to remove and select Remove. You can also right click the track when you initially remove the instrument and select "Remove track and instrument" when you delete it, if you know it's not being used by another track.