select/delete one voice over multiple measures

Select the range of measures the Right click on a note within the range ote note and use Select all similar, within range, followed by Delete.

Or

View>Selection filter and in the filter deselect everything except the voice you want to delete. Then click on first note of range, shift + click on last note of range followed by delete. Then reset the selection filter.

See https://musescore.org/en/handbook/4/selecting-elements for more ways to select things.