How can I kill/flush callbacks queued in a GUI environment?

I recreated the problem using the sample code below.

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function[] = sample_gui()

S.fh = figure('units','pixels', 'position', [700, 700, 500, 500]);

S.button_1 = uicontrol('Style', 'pushbutton', 'String', 'BUTTON_1', 'Position', [20 200 100 20], 'Callback', @button_1_call);

S.button_2 = uicontrol('Style', 'pushbutton', 'String', 'BUTTON_2', 'Position', [20 100 100 20], 'Callback', @button_2_call);

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function[] = button_1_call(varargin)

fprintf('Button 1 was pressed\n');
for p = 1:5
  button_press = true;
  while button_press
    what_is_clicked = waitforbuttonpress;
    if what_is_clicked > 0, button_press = false; end
  end
  character = get(gcf, 'CurrentCharacter');
  fprintf('%d of 5 pressed %s\n', p, character);
end

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function[] = button_2_call(varargin)

fprintf('Button 2 was pressed\n');

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Note that button 1 callback function executes for a fixed number of iterations (5 in this case) and CAN be interrupted by button 2. When I launch this GUI and execute button presses in the same order: button 1, I execute just three of the five iterations, interrupt by button 2, then again press button 1 and execute all five iterations.

Desired Output
----------------------------
Button 1 was pressed
1 of 5 pressed a
2 of 5 pressed b
3 of 5 pressed c
Button 2 was pressed
Button 1 was pressed
1 of 5 pressed a
2 of 5 pressed b
3 of 5 pressed c
4 of 5 pressed d
5 of 5 pressed e

Observed output
----------------------------
Button 1 was pressed
1 of 5 pressed a
2 of 5 pressed b
3 of 5 pressed c
Button 2 was pressed
Button 1 was pressed
1 of 5 pressed a
2 of 5 pressed b
3 of 5 pressed c
4 of 5 pressed d
5 of 5 pressed e
4 of 5 pressed e
5 of 5 pressed f

What bothers me is the last two lines, these are residue from my previous press of button 1. I have tried drawnow, drawnow update expose, break, return, refresh, BusyAction - 'cancel', etc in all possible permutations. None of these statements causes the for loop to exit.