how to ignore backsapce key event on input method?

According to this issue  https://github.com/jwilm/alacritty/issues/1606, I try look into Alacritty source code,and I found out that Alacritty accpet KeyboardInput directly. I want to know how to ignore backspace KeyboardInput event on input method OR how to recognize the event has been caputure by  input method ? thanks