When selecting "No instrument" in the Instruments combo box of the Pattern page, the Apply Instrument keypress does not erase instrument assignments from existing notes. Shouldn't it, logically?
IOW, with "No Instrument" selected in the listbox, selecting a group of notes (with instruments already assigned) and using the Assign Instrument shortcut does not assign "no instrument" to the notes. (The contextual menu works fine, but needs a key assignment capability.)
The behaviour should be more consistent now, i.e. "Set Instrument" also works if "No Instrument" is chosen: http://sagagames.de/stuff/mptrack.exe