Just enter 0 in one field and then right-click => "map all notes to sample 0". I don't think that we need an extra menu entry for "mapp all notes to no sample".

True, we don't need an extra menu option. But since most of the time when we assign plugins to an instrument, we don't intend to use a sample slot with it. Perhaps there should be a General Option to not map a sample to an instrument when a plugin is chosen for it (UNLESS the sample map has already been modified from its default)...

That does also exist already. Just add a VSTi, open its editor, press a note key and OpenMPT will ask you if it should create an instrument - if you agree, the instrument will have an empty note map. The same functionality can also be archieved by using "Create instrument from plugin" from the plugin editor's File menu (this was supposed to be a feature in OpenMPT 1.19 final but I forgot to commit a file so it's just in the latest stuff versions). That are truly enough ways to create instruments from VSTis in my eyes.

I already know the ways of clear note mapping, but I am just talking about if you create an instrument in a previously edited song, with less samples than instrumnts, the new creation of instrument assigns the last sample. I have a song (for example) 12 instruments and 7 samples. 5 of them were VSTi. If you click on the 'Add New Instrument' button, the new instrument sets it sample to sample 7. This useful while you edit songs. Maybe a keyboard shortcut, can do this or Ctrl or Alt plus Click.