Lasso selection and operations on chord notes

I'm working with input from a MIDI file with some obviously-incorrect very high notes in almost every chord. Since the range of the treble staff and below is roughly correct, the natural thing to do seemed to be a lasso drag selection of the upper "dodgy" registers and delete them (or replace with a rest if necessary).

This does not seem possible however, at least given my extremely rudimentary familiarity with Musescore. The notes are selected, but the unselected notes in those chords (which I want to keep) are then surrounded by a blue rectangle, and operations like cut and transpose seem to affect the entire chords.

I can manually select each of the errant notes by cmd-clicking (on my Mac, probably ctrl-clicking on Windows/Linux) and then deleting them, but this is extremely tedious and error prone (i.e. accidentally clicking the wrong thing and causing all the notes to be deselected). Is there any way to select _only_ the notes bounded by the lasso drag? I've tried cmd-shift clicking and a few other things but nothing has helped.