Click the .seq (Sequencer) button in Biab. Find the track and delete it within the .seq function. The .seq will only work on an imported midi file.

FYI: If you wanted to delete a Biab MIDI style bass track, simply r-click on the bass name in the radio bar until it turns red. Red means the part has been muted and should not be rendered when making a midi file from within the Biab .Mid application render button.
Trax