Suggested here:
https://forum.image.sc/t/can-i-remove-smooth-shortcut-key-ctrl-shift-s-of-imagej-fiji/42922
by Jerome Mutterer


import ij.*;
import ij.plugin.*;

public class Remove_Smooth_Shortcut implements PlugIn, CommandListener {

   public void run(String arg) {
      if (IJ.versionLessThan("1.39l")) return;
      Executer.addCommandListener(this);
   }
   public String commandExecuting(String command) {
       if (command.equals("Smooth")&&IJ.shiftKeyDown())
         return("PNG..."); 
      return command;
   }
}