I found the following solution, which works for me (extract only):

Button* button1 = NULL;
// create button
if (button1) delete(button1);
button1 = new Button(0, 0, 158, 80, false);
// disable button
button1->delHandlers(_softButtonEvent);
// the following line is optional, does not change anything if button is deleted
// button1->erase(BLACK); // documentation in M5Button.h -> line 716
delete(button1);

// that trick does it; other approaches did not work reliably
button1 = new Button(0, 0, 0, 0, false, "");